streamline interface between wav_set and psi

[svdavid]

wav_set (and subclasses) should be modified to make explicit what parameters should be user-defined in the psi interface.

For complex wav_sets (which have children) collapse all user parameters into a single list(?)

parameters should be specified in a list of dictionaries... indicating (variable name, text label, variable type, default value)

[svdavid]

seems to be resolved now in the new_parameters branch.

wav_set classes should define a default_params dictionary, which is used to general the enaml structures for the UI. FgBgSet provides the model format. Implemented and tested in VGN. Still pending for CAT.