Merge PACs stuff with flows for processes

[alexdewar]

Description

Every process has at least one commodity flow, at least one of which must be designated as a PAC (primary activity commodity). Currently this information is contained in two input files: process_flows.csv and process_pacs.csv. The latter just contains pairs of process and commodity IDs indicating which of the commodity flows are PACs for each process. The code reflects this input file structure: the Process struct has separate flows and pacs fields, which isn't particularly usable. It means if you want to find out whether a given commodity flow is a PAC you have to iterate over Process.pacs to check if it's in there. Conversely, to get the process flows corresponding to PACs, you have to find the flows whose commodity IDs match those in Process::pacs.

While the code doesn't actually use these data structures for anything other than validation yet, when we do come to use them (we need them for the dispatch optimisation), this will be annoying. We can simplify things by just adding a boolean flag to each process flow indicating whether or not it's a PAC. We can then delete the process_pacs.csv example file and most of the associated validation code, which is what I've done. I've moved the remaining PAC validation steps to src/input/process/flow.rs.

Unrelated change: I also tidied up some doc comments in src/process.rs.

Closes #337. Fixes #338.

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 96.37681% with 5 lines in your changes missing coverage. Please review.

Project coverage is 95.74%. Comparing base (a750026) to head (6df67e7).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/process.rs	0.00%	3 Missing ⚠️
src/input/process/flow.rs	98.51%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #352      +/-   ##
==========================================
+ Coverage   95.67%   95.74%   +0.07%     
==========================================
  Files          28       27       -1     
  Lines        3211     3129      -82     
  Branches     3211     3129      -82     
==========================================
- Hits         3072     2996      -76     
+ Misses         62       61       -1     
+ Partials       77       72       -5     

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[tsmbland]

Cool, was about to start #248, so will build off this

[tsmbland]

Looks good!