clean up and document coupler fields

[LenkaNovak]

We need to ensure that all coupler fields have sensible names, which are documented in our docs.

To make use of the coupler as easy as possible for component models, we should make sure that the documentation explicitly outlines the interface we require, including functions and quantities required to run with our setup.

closed by #644 (see PR for To Do/Content lists)

To be done in separate PRs (to limit scope)

• component model files: rename to model.jl, combine bucket_init and bucket_utils -> bucket.jl
• separate surfacestub, componentmodel sim
  • move surfacestub to separate file outside of Interfacer.jl?
  • include from Interfacer.jl so it's still in that module
  • same for FieldExchanger.jl