feat(*): add Model

[david-r-cox]

This PR adds components of the MPS data model, including:

• RowColumnValueMap for storing matrix data
• Model to represent an MPS file
• Parsing logic to try to build a Model from a Parser

It also includes additional test cases and examples.

This lays the groundwork for being able to fully parse and represent MPS files. Additional pieces like validating the models and interpreting the data still need to be implemented.

[codecov-commenter]

Codecov Report

Attention: 25 lines in your changes are missing coverage. Please review.

Comparison is base (f7bcc00) 76.92% compared to head (cfc007f) 88.68%.

Files	Patch %	Lines
src/model/mod.rs	92.39%	7 Missing ⚠️
src/model/row_column_value_map.rs	83.78%	6 Missing ⚠️
src/model/bounds_map.rs	90.47%	4 Missing ⚠️
src/main.rs	0.00%	3 Missing ⚠️
src/model/row_type_map.rs	95.12%	2 Missing ⚠️
src/model/rhs_map.rs	96.55%	1 Missing ⚠️
src/parse.rs	98.33%	1 Missing ⚠️
src/types.rs	87.50%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main       #5       +/-   ##
===========================================
+ Coverage   76.92%   88.68%   +11.76%     
===========================================
  Files           4       10        +6     
  Lines         117      433      +316     
===========================================
+ Hits           90      384      +294     
- Misses         27       49       +22     

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