ENH: read_mmcr method to read ARM MMCR b1-level data files

[isilber]

A method to robustly read ARM MMCR b1-level data files (the datastream available through the ARM Archive).
The method returns a dict with keys representing each of the operated MMCR modes (available in the data files) and values representing the corresponding Radar objects. An optional input parameter (mode_to_extract) enables only extracting a single mode, in which case a single Radar object is returned.

The method is written in a similar format to the read_kazr method (which has some clutter in it) under the same aux_io module.
That module lacks unit tests (as also noted in its __init__.py script) and therefore I didn't include unit tests.

• [ x] Documentation reflects changes

[zssherman]

I left one comment, besides that looks good. @mgrover1 can give this a review as well, when you have time?

[isilber]

I just added options to read dual-pol channels' MMCR data (limited number of data files).
I also changed the PRT specifications to be read from the loaded file instead of hardwired handbook values

[mgrover1]

This looks great! Thanks @isilber for putting this together

[mgrover1]

@zssherman - did you need to take another look? Otherwise, we are good to merge!

[zssherman]

Nope, merge away! Thanks again @isilber !