Add pickle_tables argument to load functions

[hagenw]

See audeering/audformat#440 and audeering/audbcards#100

Adds pickle_tables argument to audb.load(), audb.load_to() and audb.load_table() with default value of True. If pickle_tables is True, it will store a table in its original format (csv or parquet) in the cache or in root (in the case of audb.load_to()), but also load the table ones and store as pickle file. This will speed up loading the table a second time, and was the default behavior before.

With pickle_tables is False we provide the option to load tables to the cache, that do not fit into memory, and provide the user the option to avoid loading the whole table when only a few lines should be read.

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[ChristianGeng]

It is quite clear what happened in this merge request.
Minor wording and documentation bumps have been adressed.

Approving.