Add diagnostics to solo dycore

[oelbert]

Is your feature request related to a problem? Please describe.
It's hard to develop pyFV3 outside of Pace because the diag manager lives in Pace, so any time you try to examine a new test case or step through the model you only have access to the state inside a parallel context and at the current timestep

Describe the solution you'd like
Some level of diagnostic handling in the dycore itself