Process input tables simplification

[RobinL]

A small PR setting some of the foundations to do analysis of blocking rule without a linker.

This is a step towards solving #2142

Since the process_input_tables function is used only during table registration (and is part of the table registration logic) it makes senes to perform is as part of table registration as opposed to a separate function call.

This means that e.g. in profile_columns we need a single register_multiple_tables call rather than three lines:

tables = ensure_is_list(table_or_tables)
tables = db_api.process_input_tables(tables)
splink_df_dict = db_api.register_multiple_tables(tables)

We'll need similar logic for any functions that do exploratory analysis without a linker, so we don't want to have to repeatedly write this logic.

Don't know why the docs build triggered here, but all the other tests are passing

[ADBond]

Great, seems sensible.

[ADBond]

I think we don't need this call, as it will be handled in register_multiple_tables

[RobinL]

Good spot thanks