Give process taxa a UID

[leander-j]

Currently only entities have a UID. This UID is (among other cases) used when saving the trajectory to a pickle file. Variables for each instance of the entity are saved in a dictionary.

However, this does not work for variables belonging to a process taxon. Here, instead of e.g. Individual[UID=123] the dictionary key will be set as something similar to <pycopancore.models.social_movement_growth.Culture object at 0x7f8aa7f5d940>. In my understanding this is the same for all functions that use __str__ or __repr__ such as print-outs.

Providing a proper string/representation is not only more human-readable but also more practical for im- and exporting data.