Added PatternCoordinator class which returns a set of coordinated pattern generators

A PatternCoordinator is a set of coordinated pattern generators.
The pattern generators are named according to the labels parameter.
The dimensions to be returned are specified with the dims parameter.
PatternGenerators are first instantiated with the parameters specified in input_parameters, and then subclasses of ValueCoordinator are applied to modify the properties of these generators.
Hmm; I guess so.  Also, if we've defined these things as features, then we have to change metafeature_mapping and metafeature_fns, since they work for features, whether they are meta (like disparity) or non-meta (like orientation).  But we can't say feature_mapping, because that would be very confusing with the FeatureMapper package, given that FeatureMapper's features are slightly different, and FeatureMapper's mapping is totally different.  Maybe rename metafeature_mapping to feature_coordinators, and metafeature_fns to feature_coordinators_to_apply (which is ok to have as a long and verbose name, since it is just an attribute and shouldn't appear in any docstring or other user documentation.