Get a list of parameters and vertical levels by reading the bodies of a minimal set of .idx files

[JackKelly]

Get a list of parameters and vertical levels by:

• reading the bodies of the first day's worth of .idx files, and the bodies of the last day's worth of .idx files.
• If the first and last days have the same coordinate labels then assume that the coordinate labels stay the same across the entire dataset, and we're done.
• If the coords in the first and last days differ then begin an "over-eager" binary search of the .idx files to find when coordinates change (e.g. when the NWP is upgraded an more ensemble members are added - see NWPs change shape over time. How to detect, store, and handle these changes? #15). Submit many GET requests at once. The coords might change more than once.

Beware that, for example, the GEFS analysis step doesn't include the same parameters as the forecast steps! (Which is why it's important to read an entire day's worth of data).

Ignore step in the body of the .idx file. It's easier to get the step from the filename! (for GEFS, at least!). See #21