Move CSV-related process code to input module

[alexdewar]

Description

This PR continues the refactoring work to move all CSV-related code into submodules of input by doing this for process.rs. The code has been further split into various submodules of input::process for clarity.

The last commit (147729c) is the only one that actually changes anything (though it should still be a non-functional change). Everything else is just to do with moving code.

Closes #286.

Type of change

• Bug fix (non-breaking change to fix an issue)
• New feature (non-breaking change to add functionality)
• Refactoring (non-breaking, non-functional change to improve maintainability)
• Optimization (non-breaking change to speed up the code)
• Breaking change (whatever its nature)
• Documentation (improve or add documentation)

Key checklist

• All tests pass: $ cargo test
• The documentation builds and looks OK: $ cargo doc

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

Attention: Patch coverage is 95.49795% with 33 lines in your changes missing coverage. Please review.

Project coverage is 95.25%. Comparing base (574e1f4) to head (147729c).
Report is 18 commits behind head on main.

Files with missing lines	Patch %	Lines
src/input/process/parameter.rs	96.85%	5 Missing and 5 partials ⚠️
src/input/process/pac.rs	94.91%	4 Missing and 5 partials ⚠️
src/input/process.rs	85.71%	0 Missing and 6 partials ⚠️
src/input/process/availability.rs	82.85%	2 Missing and 4 partials ⚠️
src/input/process/flow.rs	98.67%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #296      +/-   ##
==========================================
+ Coverage   95.15%   95.25%   +0.10%     
==========================================
  Files          19       25       +6     
  Lines        2619     2615       -4     
  Branches     2619     2615       -4     
==========================================
- Hits         2492     2491       -1     
+ Misses         50       48       -2     
+ Partials       77       76       -1     

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[TinyMarsh]

Cool! Admittedly I haven't scrutinised every line change here so this is a passing review but it looks good to me, and I have built and tested and had no problems running.

[alexdewar]

Great! Thanks for testing 😄

[alexdewar]

I'm going to merge this even though it only has one review as the changes are non-functional and are broadly in line with what was agreed in the meeting, so that I can start working on some other things on top of this. We can always tweak things later.