handle 413 errors in point selection

```python
coords[1:3]
Out[100]: [(441, 82852), (441, 88209)]
len(coords)
Out[101]: 2500

data = ds_remote[coords]
Traceback (most recent call last):
File "/home/wjiang2/.local/lib/python3.6/site-packages/IPython/core/interactiveshell.py", line 2910, in run_code
exec(code_obj, self.user_global_ns, self.user_ns)
File "", line 1, in
data = ds_remote[coords]
File "/home/wjiang2/.local/lib/python3.6/site-packages/h5pyd/_hl/dataset.py", line 848, in __getitem__
rsp = self.POST(req, body=body)
File "/home/wjiang2/.local/lib/python3.6/site-packages/h5pyd/_hl/base.py", line 477, in POST
raise IOError(rsp.reason)
OSError: Request Entity Too Large
```