Move smaller MRS bits to components.py.
Add more tests for Xmrs components.

Almost all components are covered fully, now.

Also, LnkMixin now works even if the class does not have a 'lnk' attribute.
Move some internal config values to classes.

Also renamed the mrs_test.py unittest to mrs_components_test.py.
Actually remove Lnk vars from .../mrs/config.py
Move Argument and Pred vals from config to classes
Move non-essential functions out of classes.