Add new model and test scaffolds

[cc-a]

Description

This PR adds the data files for the planned changes to the data input (created by @tsmbland) as a new example model and scaffolds several tests that cover the commodities section of the planned data schema (also created by @tsmbland).

It's expected that the test functions will have to be customised slightly based on the different implementations we try but the datastructures they check shouldn't change much.

Putting this against the new-data-input branch in lieu of develop so we have somewhere to collate our experimentation before attempting to put anything into develop.

Fixes # (issue)

Type of change

Please add a line in the relevant section of
CHANGELOG.md to
document the change (include PR #) - note reverse order of PR #s.

• New feature (non-breaking change which adds functionality)
• Optimization (non-breaking, back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (whatever its nature)

Key checklist

• All tests pass: $ python -m pytest
• The documentation builds and looks OK: $ python -m sphinx -b html docs docs/build

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 33.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 71.38%. Comparing base (e09a23d) to head (eb68180).

❗ Current head eb68180 differs from pull request most recent head 935b3e2

Please upload reports for the commit 935b3e2 to get more accurate results.

Files	Patch %	Lines
src/muse/examples.py	33.33%	4 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##           new-data-input     #373      +/-   ##
==================================================
- Coverage           71.39%   71.38%   -0.01%     
==================================================
  Files                  44       44              
  Lines                5887     5892       +5     
  Branches             1162     1163       +1     
==================================================
+ Hits                 4203     4206       +3     
- Misses               1364     1367       +3     
+ Partials              320      319       -1     

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[alexdewar]

LGTM, modulo a couple of comments.

[alexdewar]

Why is year a string here?

[alexdewar]

Can't we just use the default value for abs?

Suggested change

	- 14.81481,
	abs=1e-4,
	- 14.81481