Move push_pull!.jl functionality to src/

[LenkaNovak]

To be able to update for the new ClimaAtmos release, we needed to change our flux calculation and field passing interface. This is done here by moving and generalizing the push_pull!.jl code to src by setting up a FieldExchange and FluxCalculator modules. This also the last step needed before implementing the partial flux calculation PR.

Components

• setup FieldExchange and FluxCalculator code (non-modularized - easier testing)
• ensure slab planet and AMIP run as in the main
• ensure AMIP runs
• modularize
• add unit tests
• add docs

QA

• reproducible results compared to Buildkite (small changes expected, due to different order of model initialization)

• no substantial performance deterioration