Add category id cache.

[riga]

This PR adds a persistent caching mechanism for category ids by category name, and adjusts the add_category() and create_category_combinations() config utils to use it.

With that, the pattern of adding categories can become

from columnflow.config_util import CategoryIDCache, add_category

id_cache = CategoryIDCache(config_inst).open()

add_category(name="etau", selection="cat_etau", id=id_cache)  # users might use partial :)
...  # more categories here

id_cache.close()

The cache makes sure to use an increasing id for yet unseen category names, and the previously seen id otherwise. Upon close(), the cache is saved to a common location.