First pass at duckdb data interface

[cc-a]

Description

A first mocked up version of what a revised data input layer may look like using standalone (i.e. no sqlalchemy) duckdb. So far implemented:

• reader functions for the region, commodity definition and commodity demand tables that populate the their data into duckdb whilst also returning a numpy array representation.
• calculate_global_commodities function that takes the data from the commodity definition table and transforms it into the xarray representation required by Muse internally.
• tests for each function implemented plus additional tests addressing any constraints applied to database columns.

Mostly pushing this for visibility at this point as @tsmbland is planning to expand this to cover the remaining database tables and xarray data structures.

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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 71.30%. Comparing base (f5bec10) to head (ae61de8).

Additional details and impacted files

@@               Coverage Diff               @@
##           new-data-input     #379   +/-   ##
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  Coverage           71.30%   71.30%           
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  Files                  44       44           
  Lines                5890     5890           
  Branches             1155     1155           
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  Hits                 4200     4200           
  Misses               1369     1369           
  Partials              321      321           

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