Rename / remove dense and sparse variables and refactor

[timmens]

• Rename sparse variables to combination variables
• Rename dense variables to variables, or product variables
• Update return type of _determine_dense_discrete_choice_axes
• Replace double backticks with single backticks

Future PRs:

• Update explanation notebooks and run again in GHA
• Currently we have discrete_problem._determine_dense_discrete_choice_axes and simulate.determine_discrete_dense_choice_axes: They look similar, and do similar things, but not exactly -> Combine these with an argument like purpose: Literal["solution", "simulation"].

[hmgaudecker]

Very neat!

I think type annotations throughout would really help. Especially getting rid of None instances where empty containers will just do the trick and allow simplifying the code. No need to include here, but probably a good one to tackle soon.

[timmens]

As discussed, we will further split the code-base into parts that are relevant only for simulation or solution. For this we will also split spacemap into two functions; therefore we won't resolve the comments on dispatchers.py here, but in a separate PR.

[hmgaudecker]

Sweet!

[hmgaudecker]

👍