EAMxx: intelligently handle fillval tracking in diags

Diagnostics being their own derived classes are supposed to handle variety of inputs/outputs.

- Sometimes diagnostics can produce their own fillval depending on conditions. Take FieldAtXyz being unavailable on a mountain or something. Another is OpticalDepth not being available at night.
- Other time, diagnostics must take into them masked fields, and thus will have to reckon with these masked/fill values.

In both cases, diagnostics are supposed to be able to cleanly and intelligently handle accumulation. Currently, only the FieldAtXyz is handled okay.

Options:
1. Make all diagnostics redundantly track accumulation. This is not desirable as it is a blanket solution
2. Introduce a functionality to determine if a diagnostic needs to track the accumulation carefully based on the conditions above (i.e., it either takes in a field that requires careful handling or that it itself produces fillvals thus needing careful handling)