[python] Get rid of use_theoretical_model

[duburcqa]

We should get rid of use_theoretical_model everywhere to make everything more simple. The user would be responsible for converting the state for flexible to rigid or the other way around if necessary.

[duburcqa]

Note that it is still relevant to create theoretical visual and collision models (at least internally), as they are used to generate the actual visual and collision models later on. Making them private would be an option, but it would require to move the "serialization" in Model itself, which is not ideal for now because of design flaws.

[duburcqa]

Fixed by #753