Add tests

[callumrollo]

The simplest test could make use of http://xarray.pydata.org/en/stable/generated/xarray.DataArray.equals.html to check that the final datasets do not change. This would require uploading a finished dataset to the repo. The timeseries dataset is the smallest, would make a good target

Eventually tests should cover all of the functions in pyglider. This will take time, but we should try to ensure that all new functionality comes with tests

[callumrollo]

Idea for test dataset: upload it in a seperate repo and use xarray's remote netcdf load to download it and test against it. This sidesteps the problem of adding bulky test data to this repo. This will be especially useful if the netcdf format evolves down the line (Issue #21). Having our test data external to the main repo would prevent adding a whole netCDFs worth of bulk to git each time a change is made to an attribute name