Compute and plot average diagnostics from existing ones

[amorison]

One should be able to compute and plot diagnostics that corresponds to trivial operations on existing diagnostics. Such as:

• radial profiles computed from complete fields;
• time series computed from radial profiles or complete fields.

proposed syntax

For example, if one has a compositional field, say cX, whose average isn't in rprof data, one could plot its radial profile with stagpy rprof -o=<cX>. Ideally, names of the form <X> should be usable where a radial profile name is expected as a CLI argument or in the API.