Read in commodities-related CSV files

[alexdewar]

Description

The latest installment of the input layer work. This one is minimally invasive; it just adds support for two commodity-related CSV files (commodities.csv and commodity_costs.csv).

The commodity constraints file (which is currently empty in the example) is not read in by this code, but I've opened an issue to tackle it in future: #149.

Closes #62.

Type of change

• [ ] Bug fix (non-breaking change to fix an issue)
• [x] 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

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

Further checks

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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.75510% with 6 lines in your changes missing coverage. Please review.

Project coverage is 84.96%. Comparing base (cd6d92c) to head (9fd6c20). Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
src/model.rs	33.33%	4 Missing :warning:
src/lib.rs	0.00%	1 Missing :warning:
src/time_slice.rs	87.50%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #150 +/- ## ========================================== - Coverage 85.20% 84.96% -0.25% ========================================== Files 11 12 +1 Lines 338 379 +41 ========================================== + Hits 288 322 +34 - Misses 50 57 +7 ```

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[alexdewar]

I'm not entirely sure how the timeslices will work in practice, though, now that they can be provided also as a season level only.

I guess we can cross that bridge when we come to it :smile:. We will need to decide what to do e.g. if a user specifies a specific time slice (winter.evening) when there's already a cost specified for the whole season (winter) at some point. I've opened an issue for this: #153