Non-strict var name check

[ThomSerg]

Added some internal tools to have greater control over variable name checks.
When working on file readers and writers, I encountered the issue that we couldn't read the model files that we ourselves have exported, due to the use of IV and BV decision variable namings. In this PR I added a couple of helper functions to allow for these variable names as long as they haven't been used before.

Now, file readers can wrap their logic in

with ignore_strict_variable_name_check():
    ... create CPMpy model based on file contents here ...

This will temporarily allow the use of restricted variable names (but still checks for variable name collisions).

Additionally I also extracted the "update variable counters from model" logic used when loading models from .pickle files to make it reusable (_update_variable_counters) (file readers will also need this)

For now, readers will still have to update the counters themselves after exiting ignore_strict_variable_name_check, i.e. after creating the CPMpy model they should call:

_update_variable_counters(model)

But I might have found a way to do this automatically (that could potentially also resolve the issues when copying CPMpy models across processes, but to be seen / no promises) (different approach than Ignace's). Will be for a follow-up PR.

[IgnaceBleukx]

Very elegant use of the context manager : )
I would just make the function call also private, to really indicate this is non-trivial usage of the library.

[IgnaceBleukx]

So for that you would need access to the model here?

[ThomSerg]

Yes, with the current update function you would indeed. So currently the only approach to do it somewhat automatic is to create you model before entering the context, and then call:

with ignore_strict_variable_name_check(model=model):
    ...

The context manager then has the model and can call the update for you on exit.

[tias]

fine by me...

will also be of benefit in other file readers I understand?

If Ignace has no further comments then can go in...

maybe do add your example from the comment in the Docstring