(Towards #2381) Re-structure classes for reduction nodes.

[arporter]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.95%. Comparing base (8ecb098) to head (2730335).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3333      +/-   ##
==========================================
- Coverage   99.95%   99.95%   -0.01%     
==========================================
  Files         382      384       +2     
  Lines       54094    54009      -85     
==========================================
- Hits        54072    53987      -85     
  Misses         22       22              

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[arporter]

ITs were green. This is ready for a first look from either @hiker, @sergisiso or @LonelyCat124. There's a fair bit of code movement as I've taken code out of the monoliths that are lfric.py and psyGen.py and put it into separate modules. I've done the same thing for the tests. Essentially though, it just restructures the class hierarchy and introduces LFRicGlobal{Max,Min,Sum}.