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.
Here input_type becomes a regular Python attribute, overshadowing the Parameter defined in the superclass, which is not what was intended.  To inherit the various Parameter elements specified in the superclass, need to declare it as a Parameter of the same type here, and then supply a new default (or anything else you want to override): input_type=ClassSelector(default=FileImage).