xarray runtime warning

Right now we are ignoring this warning on load/save but it should be investigated:

```Traceback (most recent call last):
File "/home/guyn/Dropbox/python/AstroRetriever/src/project.py", line 966, in run
self.analysis.analyze_sources(source_batch)
File "/home/guyn/Dropbox/python/AstroRetriever/src/analysis.py", line 304, in analyze_sources
self._save_histograms(temp=True)
File "/home/guyn/Dropbox/python/AstroRetriever/src/analysis.py", line 668, in _save_histograms
hist.save(suffix=suffix)
File "/home/guyn/Dropbox/python/AstroRetriever/src/histogram.py", line 964, in save
self.data.to_netcdf(fullname, mode="w")
File "/home/guyn/Dropbox/python/AstroRetriever/venv/lib/python3.8/site-packages/xarray/core/dataset.py", line 1912, in to_netcdf
return to_netcdf( # type: ignore # mypy cannot resolve the overloads:(
File "/home/guyn/Dropbox/python/AstroRetriever/venv/lib/python3.8/site-packages/xarray/backends/api.py", line 1178, in to_netcdf
engine = _get_default_engine(path_or_file)
File "/home/guyn/Dropbox/python/AstroRetriever/venv/lib/python3.8/site-packages/xarray/backends/api.py", line 141, in _get_default_engine
return _get_default_engine_netcdf()
File "/home/guyn/Dropbox/python/AstroRetriever/venv/lib/python3.8/site-packages/xarray/backends/api.py", line 119, in _get_default_engine_netcdf
import netCDF4 # noqa: F401
File "/home/guyn/Dropbox/python/AstroRetriever/venv/lib/python3.8/site-packages/netCDF4/__init__.py", line 3, in
from ._netCDF4 import *
File "src/netCDF4/_netCDF4.pyx", line 1, in init netCDF4._netCDF4
RuntimeWarning: numpy.ndarray size changed, may indicate binary incompatibility. Expected 16 from C header, got 96 from PyObject
```