Add diagnostic_name metadata field

[peverwhee]

Description

Add new diagnostics name metadata field to be used by the constituents object*

*Note: for now, the new metadata field will only be used by the constituents object, but can be extended to be used by the amorphous future diagnostics object (#579)

Related to #461

Solution

1. Add new optional metadata field - diagnostic_name - to the metadata.
2. Add new diagnostics name attribute to the constituents object that is set to the diagnostic name field in the metadata (or through the instantiate call)

[gold2718]

How is this diagnostic_name used? For a single constituent, there may be many places where the value of a constituent may want to be sampled. Is this name just for host model use? Is it a base name for other uses (e.g., <diagnostic_name>_after_dynamics, <diagnostic_name>_before_microphysics)?

[peverwhee]

For now at least, it will just be host model use. It'll be up to the host model to add _after_dynamics if necessary (so, I guess just a base name as you say).