Usage of default value for new member.

Summary:
If the passed value for a new member equals 'None', we set the value to the default value.
Added test for setting of default value for _add_member

Signed-off-by: Andre Klein <andre.klein@dkfz-heidelberg.de>

Test Plan: Added a test case to test_hyppopysolver

Reviewers: floca

Reviewed By: floca

Differential Revision: https://phabricator.mitk.org/D499