ENH: Bring in extended list of supported message structures from MetPy for Level2 reader

[jthielen]

As referenced in #158 and discussed with @mgrover1 in the MetPy telecon yesterday, there is still some relevant code in main...jthielen:xradar:nexrad-level2 that may be helpful, namely that of a reconciled list of message structures combining those from both Py-ART's and MetPy's. Given that prior effort used MetPy's bespoke NamedStruct and BitField objects to handle this, there would likely be a quite a bit of refactoring needed (going either direction..modifying the reconciled lists to match the current xradar approach, or updating the xradar approach to use these MetPy-style objects). This issue can hopefully track this particular facet of the overall effort of #40.

[kmuehlbauer]

Thanks @jthielen for pushing this here.

I can have a look what would be necessary to incorporate things like BitField into the current structure.

I'm also interested to check if and how NamedStruct can be utilized for the current processing.