Add default value for grouped_parameters.

Issues: #4
Add grouped_parameters property to Parameters.

This adds the appropriate tests of and implementation that provides the
grouped_parameters property requested in issue #4.
Add python warnings to correlate logged warnings.

Not only do we use logging to record warnings (handled by the
applications logging configuration) but also use Python's warning system
to allow those warnings voice.
Add doctest checks into setup.cfg.

The testable examples written into the test_fixtures module should be
checked as well as the actual tests of the module (testing the tests).