Refactoring: split process() into subcomponents

[truth-quark]

Background: the process() workflow function is ~140 SLOC, covering multiple processing steps. Testing as a whole is too difficult, given the volume of logic and integration. Work on the func should generally include:

1. Split into sub functions to facilitate unit testing
2. Retrofit unit tests
3. Refactor/clean processing logic for simplification
4. Reorder code blocks to group like functionality
5. Refactor to separate cube manipulation & cube I/O steps (e.g. split into filtering, cube modification, writing cubes)

This helps with modularity, readability, workflow modification & future expansion.

Tasks:

• [ ] Expand args (results in large method signature, then split into workflow steps)
• [ ] Refactor cmd line args to function signature args
• [x] Extract code blocks in process() to individual functions
  • [X] Mule fields files code block to func
  • [x] Heaviside uv & t block
  • [x] Cube filtering, can this be done at the start/ignore processing certain cubes?
  • [x] Add history attributes
  • [x] cube renaming
  • [x] Fix/modify units
  • [x] xconv fix / long name
  • [x] iris naming fixes
  • [X] Fix cell methods
  • [x] fix lat lon coords
  • [x] heaviside_uv masking
  • [x] heaviside_t masking
• [ ] Add unit tests for individual components
• [x] Refactor/reorder I/O steps
  • [x] gather input I/O func calls
  • [x] group cube modification func calls
  • [x] gather output I/O calls at end of process()
• [ ] Convert magic numbers to constants
• [ ] Replace print()s with warnings.warn()

[truth-quark]

Adding link to related ESM 1.5 project issue: https://github.com/ACCESS-NRI/access-esm1.5-configs/issues/3

[truth-quark]

@blimlim & @jo-basevi I've updated this high level issue to better explain the work behind process().

Does it make sense & is anything missing?