Need new diagnostic scheme to provide total precipitation fields

In at least the CAM physics suites, precipitation is split across various different variables, such as rain and snow from deep convection, shallow convection, and stratiform clouds. However, many users simply want the total surface precipitation (i.e. `PRECT` in CAM), and don't need all the separate precip fields.

Thus in order to get `PRECT` and its snow equivalent in SIMA, we'll need to create a new diagnostic scheme which can provide this to the history files.

Start by tracing how CAM physics suites expose rain and snow diagnostics to SIMA history files; the issue names PRECT and its snow equivalent as the target fields but does not identify files or tests. Done means a new diagnostic scheme provides total surface precipitation and total snowfall in the history files.

Written by the indexing model from the issue text.