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.
Change "must" to "should"?  E.g. should we add defaults for these entries? The name could default to basename(dataset_name), the length could default to the number of files in the directory, the description could default to "", the source could default to dataset_name, the filename_template could default to "{current_image}.jpg', inherent_dims could default to whatever gives xy and orientation and sf.  Some of these are useful, but I'm not sure if they all are.