Get the horizontal spatial coordinates

[JackKelly]

• Read a day's worth of GRIB files at the start of the dataset, and a day's worth at the end of the dataset.
• If the start and end of the dataset have the same coordinates then we're done; let's assume the spatial coords stay the same across the dataset.
• Otherwise conduct a kind of over-eager binary search to find exactly where the horizontal spatial coords change. The coords might change more than once.

If the scan of .idx files found that the ensemble members and/or vertical levels changed then there's a good chance that the spatial coords also changed at the same times.

[JackKelly]

I'm considering implementing this in Python. See JackKelly/hypergrib#28!