Intake-STAC with NASA CMR STAC proxy: Authentication

As part of [STAC-sprint 6](https://github.com/radiantearth/community-sprints/blob/master/08182020-remote/software/progress.md) I was trying out intake-stac with https://github.com/nasa/cmr-stac. It would be absolutely amazing to integrate intake-stac with that endpoint to facilitate working with NASA datasets! But there multiple things to work out. First and foremost is how to deal with Authentication.

Unlike boto3 cloud credentials, NASA uses and 'Earthdata login' (https://urs.earthdata.nasa.gov/documentation). Typically, science users keep their username and password in a ~/.netrc file for any time you try to retrieve a file. This mechanism doesn't currently work with the intake-stac .to_dask() method. For example:

```
item['data'].metadata
#{'href': 'https://grfn.asf.alaska.edu/door/download/S1-GUNW-A-R-087-tops-20141023_20141011-153856-27545N_25464N-PP-1a1a-v2_0_2.nc'}
da = item['data'].to_dask()
```

Leads to a big traceback:
```pytb
---------------------------------------------------------------------------
KeyError Traceback (most recent call last)
~/miniconda3/envs/intake-stac-gui/lib/python3.7/site-packages/xarray/backends/file_manager.py in _acquire_with_cache_info(self, needs_lock)
197 try:
--> 198 file = self._cache[self._key]
199 except KeyError:

~/miniconda3/envs/intake-stac-gui/lib/python3.7/site-packages/xarray/backends/lru_cache.py in __getitem__(self, key)
52 with self._lock:
---> 53 value = self._cache[key]
54 self._cache.move_to_end(key)

KeyError: [, ('https://grfn.asf.alaska.edu/door/download/S1-GUNW-A-R-087-tops-20141023_20141011-153856-27545N_25464N-PP-1a1a-v2_0_2.nc',), 'r', (('clobber', True), ('diskless', False), ('format', 'NETCDF4'), ('persist', False))]

During handling of the above exception, another exception occurred:

OSError Traceback (most recent call last)
in
----> 1 da = item['data'].to_dask()

~/miniconda3/envs/intake-stac-gui/lib/python3.7/site-packages/intake_xarray/base.py in to_dask(self)
67 def to_dask(self):
68 """Return xarray object where variables are dask arrays"""
---> 69 return self.read_chunked()
70
71 def close(self):

~/miniconda3/envs/intake-stac-gui/lib/python3.7/site-packages/intake_xarray/base.py in read_chunked(self)
42 def read_chunked(self):
43 """Return xarray object (which will have chunks)"""
---> 44 self._load_metadata()
45 return self._ds
46

~/miniconda3/envs/intake-stac-gui/lib/python3.7/site-packages/intake/source/base.py in _load_metadata(self)
124 """load metadata only if needed"""
125 if self._schema is None:
--> 126 self._schema = self._get_schema()
127 self.datashape = self._schema.datashape
128 self.dtype = self._schema.dtype

~/miniconda3/envs/intake-stac-gui/lib/python3.7/site-packages/intake_xarray/base.py in _get_schema(self)
16
17 if self._ds is None:
---> 18 self._open_dataset()
19
20 metadata = {

~/miniconda3/envs/intake-stac-gui/lib/python3.7/site-packages/intake_xarray/netcdf.py in _open_dataset(self)
56 _open_dataset = xr.open_dataset
57
---> 58 self._ds = _open_dataset(url, chunks=self.chunks, **kwargs)
59
60 def _add_path_to_ds(self, ds):

~/miniconda3/envs/intake-stac-gui/lib/python3.7/site-packages/xarray/backends/api.py in open_dataset(filename_or_obj, group, decode_cf, mask_and_scale, decode_times, autoclose, concat_characters, decode_coords, engine, chunks, lock, cache, drop_variables, backend_kwargs, use_cftime, decode_timedelta)
507 if engine == "netcdf4":
508 store = backends.NetCDF4DataStore.open(
--> 509 filename_or_obj, group=group, lock=lock, **backend_kwargs
510 )
511 elif engine == "scipy":

~/miniconda3/envs/intake-stac-gui/lib/python3.7/site-packages/xarray/backends/netCDF4_.py in open(cls, filename, mode, format, group, clobber, diskless, persist, lock, lock_maker, autoclose)
356 netCDF4.Dataset, filename, mode=mode, kwargs=kwargs
357 )
--> 358 return cls(manager, group=group, mode=mode, lock=lock, autoclose=autoclose)
359
360 def _acquire(self, needs_lock=True):

~/miniconda3/envs/intake-stac-gui/lib/python3.7/site-packages/xarray/backends/netCDF4_.py in __init__(self, manager, group, mode, lock, autoclose)
312 self._group = group
313 self._mode = mode
--> 314 self.format = self.ds.data_model
315 self._filename = self.ds.filepath()
316 self.is_remote = is_remote_uri(self._filename)

~/miniconda3/envs/intake-stac-gui/lib/python3.7/site-packages/xarray/backends/netCDF4_.py in ds(self)
365 @property
366 def ds(self):
--> 367 return self._acquire()
368
369 def open_store_variable(self, name, var):

~/miniconda3/envs/intake-stac-gui/lib/python3.7/site-packages/xarray/backends/netCDF4_.py in _acquire(self, needs_lock)
359
360 def _acquire(self, needs_lock=True):
--> 361 with self._manager.acquire_context(needs_lock) as root:
362 ds = _nc4_require_group(root, self._group, self._mode)
363 return ds

~/miniconda3/envs/intake-stac-gui/lib/python3.7/contextlib.py in __enter__(self)
110 del self.args, self.kwds, self.func
111 try:
--> 112 return next(self.gen)
113 except StopIteration:
114 raise RuntimeError("generator didn't yield") from None

~/miniconda3/envs/intake-stac-gui/lib/python3.7/site-packages/xarray/backends/file_manager.py in acquire_context(self, needs_lock)
184 def acquire_context(self, needs_lock=True):
185 """Context manager for acquiring a file."""
--> 186 file, cached = self._acquire_with_cache_info(needs_lock)
187 try:
188 yield file

~/miniconda3/envs/intake-stac-gui/lib/python3.7/site-packages/xarray/backends/file_manager.py in _acquire_with_cache_info(self, needs_lock)
202 kwargs = kwargs.copy()
203 kwargs["mode"] = self._mode
--> 204 file = self._opener(*self._args, **kwargs)
205 if self._mode == "w":
206 # ensure file doesn't get overriden when opened again

netCDF4/_netCDF4.pyx in netCDF4._netCDF4.Dataset.__init__()

netCDF4/_netCDF4.pyx in netCDF4._netCDF4._ensure_nc_success()

OSError: [Errno -78] NetCDF: Authorization failure: b'https://grfn.asf.alaska.edu/door/download/S1-GUNW-A-R-087-tops-20141023_20141011-153856-27545N_25464N-PP-1a1a-v2_0_2.nc'
```

Full example here: https://gist.github.com/scottyhq/04fe1e2d0b946b97228f6922cf001bbd