feat: Multiple updates to the Plexos parser

[ktehranchi]

The feature to convert quadratic to PWL (quad2pwl) will now have outputs that enforce monotonically increasing PWL functions. cvxpy dependency added.

List of changes:

• New feature_flag Quad2PWL feature that enforce monotonically increasing PWL for

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 57.22284% with 382 lines in your changes missing coverage. Please review.

Project coverage is 72.09%. Comparing base (a07253e) to head (1a7aff2). Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/r2x/parser/plexos.py	42.95%	251 Missing :warning:
src/r2x/parser/plexos_utils.py	59.89%	73 Missing :warning:
src/r2x/parser/parser_helpers.py	62.50%	27 Missing :warning:
src/r2x/parser/polars_helpers.py	75.86%	14 Missing :warning:
src/r2x/exporter/plexos.py	66.66%	5 Missing :warning:
src/r2x/exporter/handler.py	78.57%	3 Missing :warning:
src/r2x/exporter/sienna.py	91.42%	3 Missing :warning:
src/r2x/parser/handler.py	81.25%	3 Missing :warning:
src/r2x/models/costs.py	87.50%	1 Missing :warning:
src/r2x/parser/reeds.py	90.90%	1 Missing :warning:
... and 1 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #47 +/- ## ========================================== + Coverage 62.18% 72.09% +9.90% ========================================== Files 29 31 +2 Lines 3115 3472 +357 ========================================== + Hits 1937 2503 +566 + Misses 1178 969 -209 ```

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