Fixed the Params API in Python

Created by: muffato

This fixes another issue I've found with @marcoooo yesterday. self.param_required('a') is supposed to raise an exception if a is None (like in Perl). In fact, when the behaviour of the param* methods was rationalised in Perl, the decisions should have been applied to the Python implementation, but I forgot to do so. Both implementations should now be consistent and in sync with the manual: https://ensembl-hive.readthedocs.io/en/master/creating_runnables/IO_and_errors.html#parameter-handling