Add default argument to get_reactions_kcat_mapping

[yaccos]

When using get_reactions_kcat_mapping, you may not have any user-defined protein database. Hence, requiring the argument protein_kcat_database_path to have a value is therefore counterintuitive. From inspecting the function, I find that supplying an empty string as the value will make the function ignore the database. I therefore suggest having the empty string as the default argument for protein_kcat_database_path.

[Paulocracy]

I (once again :-) agree with you and added a default parameter.