Specify framework/model interface elements beyond BMI

[PhilMiller]

BMI describes an API for passing data within a single process between different models and driver code. There are a number of things that it doesn't specify, that we will need for composite execution of models within the overall NWM NextGen framework provided by ngen.

Those include

• Domain definition and discretization
• MPI
  • communicator assignment
  • problem decomposition
  • execution coordination
• Logging (see Implement a logging solution #574 comments)
• In-process parallelism
• Timestepping logic and coordination
• Dynamism in necessary input variables or available output variables

Please edit this to add to the list, and we can start linking document(s) that address points as work proceeds on them.

[mattw-nws]

Seems related to the "BMI Conventions" involved in some things we're already doing and things that I've started trying to document in #507 .