Feature: public methods to interface with new backend model instance

[brynpickering]

Fixes issue #429

Summary of changes in this pull request:

General restructuring:

• Move pyomo backend model class to its own file.
• Move all backend interface methods to be public on the backend object itself, rather than making them available as methods on the model object (e.g., "model.to_lp" -> "model.backend.to_lp"). This came after I realised just how many methods would need to be passed through to the model object (and each one would need wrapping in functions to check the backend model has been built and to copy across docstrings from the backend) - seemed like too much. We just have to get people used to the "backend" as a thing (or give it a different name).
• Store a copy of the model data in the backend so it doesn't need to be supplied every time a method is called.

New methods:

• Update parameter values
• Update variable bounds
• Fix/unfix a variable

Reviewer checklist:

• Test(s) added to cover contribution
• Documentation updated
• Changelog updated
• Coverage maintained or improved

[codecov]

Codecov Report

Merging #471 (210041e) into main (cbed4c8) will decrease coverage by 2.87%.
Report is 1 commits behind head on main.
The diff coverage is 95.59%.

❗ Current head 210041e differs from pull request most recent head 4069505. Consider uploading reports for the commit 4069505 to get more accurate results

@@            Coverage Diff             @@
##             main     #471      +/-   ##
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- Coverage   94.98%   92.12%   -2.87%     
==========================================
  Files          31       32       +1     
  Lines        3890     3975      +85     
  Branches        0      946     +946     
==========================================
- Hits         3695     3662      -33     
- Misses        195      196       +1     
- Partials        0      117     +117     

Files	Coverage Δ
src/calliope/__init__.py	100.00% <ø> (ø)
src/calliope/_version.py	100.00% <ø> (ø)
src/calliope/backend/helper_functions.py	99.35% <ø> (ø)
src/calliope/backend/where_parser.py	98.43% <ø> (ø)
src/calliope/cli.py	81.67% <100.00%> (ø)
src/calliope/core/__init__.py	100.00% <ø> (ø)
src/calliope/core/attrdict.py	99.50% <ø> (ø)
src/calliope/core/io.py	89.47% <ø> (ø)
src/calliope/core/model.py	93.23% <100.00%> (ø)
src/calliope/core/util/generate_runs.py	87.09% <ø> (ø)
... and 22 more

[sjpfenninger]

Good to go - though we should track points to revisit later, e.g. the naming of global_expressions (one alternative suggestion is to call them named_expressions)