Clean up code for Formulation in advance of Coastal introduction

[PhilMiller]

I want to slot in new code for Coastal total water level (TWL) functionality as cleanly as possible with the existing code base. In particular, it seems like CoastalFormulation should be a natural analog to the existing CatchmentFormulation. A lot of stuff there in the baseFormulation interface right now is specific to the existing lumped catchment models, or is just defunct. I try to clean out what's gotten in my way so far, to make the subsequent additions easier.

Additions

None

Removals

• Nexus Manager class stub that was added years ago and never fleshed out or used
• Scalar overload of Formulation::get_var_value_as_double, which was papering over an assumption of single element arrays that we agreed should be made more explicit

Changes

• Move various functionality that's largely specific to our implementation of lumped catchments down from the Formulation base class to the CatchmentFormulation intermediate class
• Move functionality duplicated across the several Bmi_{C,Cpp,Fortran,Py}_Formulation up to their common base Bmi_Module_Formulation class
• Move arrays of variable names describing required parameters to BMI modules to class static members, to avoid duplicate allocation per instance.

Testing

1. Local testing on macOS
2. CI

Checklist

• [x] PR has an informative and human-readable title
• [x] Changes are limited to a single goal (no scope creep)
• [x] Code can be automatically merged (no conflicts)
• [x] Code follows project standards (link if applicable)
• [x] Passes all existing automated tests
• [x] Any change in functionality is tested
• [x] New functions are documented (with a description, list of inputs, and expected output)
• [x] Placeholder code is flagged / future todos are captured in comments
• [x] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• [x] Reviewers requested with the Reviewers tool :arrow_right:

[PhilMiller]

I think this is mostly in shape to clear the path for Coastal needs. I'm still uncertain about create_formulation, but that could be addressed in a subsequent PR.