Tidy scipy adapter code

[tsmbland]

Tidying up the scipy adapter code, based on this suggestion:

I always thought this was a very odd design pattern. Why not simply storing the templates in attributes of ScipyAdapter and making the relevant methods like back_to_muse not static? It will be way simpler to follow.
All in all, and considering the size of this file, I think it will make sense to extract this as in its own module and make most of these static methods just plain functions in there. They way they are used here feels like the wrong way of using static methods, specially when you are calling them within the class itself.
Originally posted by @dalonsoa in #685 (comment)

The scipy adapter code now lives in a new file (lp_adapter.py), separate from the constraints code, and uses a simpler design pattern, as suggested.

I've also reworked the tests a bit, mostly to tidy up the fixtures and put the tests in a more logical order. I think this is an improvement. Despite splitting the lp adapter tests into a new file, I haven't split the tests into a new file. The reason is that the constraints and lp adapter tests share pretty much the same fixtures, so to avoid duplicating fixture definitions it made more sense to keep the tests in the same file. Maybe there's a better solution though - happy for suggestions

[Copilot]

Pull Request Overview

This PR extracts the LP adapter logic into a new lp_adapter.py module and updates references in investments.py to use the new adapter.

• Introduce src/muse/lp_adapter.py containing utilities to convert MUSE xarray structures for SciPy LP.
• Update src/muse/investments.py to import and call ScipyAdapter.from_muse_data from the new module.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/muse/lp_adapter.py	New module with conversion functions and adapter
src/muse/investments.py	Updated import path and factory call for adapter

Comments suppressed due to low confidence (1)

src/muse/lp_adapter.py:54

• [nitpick] The loop variable u is ambiguous. Consider renaming it to something more descriptive like var_name to improve readability.

result = data[[u for u in data.data_vars if str(u).startswith(name)]]

[dalonsoa]

I have not gone into the detail of the code, but this is way, way more readable and, specially, the code structure makes sense, facilitating understanding the workflow of the application. I would not worry too much about the structure of the tests.