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.
Oops!  I see that we have no base class common to all composite-style PatternGenerators, so you had to use PatternGenerator here instead.  I think the set of suitable classes currently consists of Composite, SeparatedComposite, and Selector, which all support the generators parameter and change the default size to 1.0.  I think we should add a new abstract PatternGenerator type for each of those classes to inherit from:

```
class CompositeBase(PatternGenerator):
    """                                                                                                                                                
    PatternGenerator that combines or selects from a list of other PatternGenerators.                                                                  
    """

    __abstract=True

    generators = param.List(class_=PatternGenerator,default=[Constant(scale=0.0)],
                            bounds=(1,None),precedence=0.97, doc="""                                                                                   
        List of patterns to combine or select from.")

    size = param.Number(default=1.0,doc="Scaling factor applied to all sub-patterns.")
```

For full backwards compatibility, Selector would then need to override the default list of generators, but given how useless its current default is, presumably no real code would ever use that default, so maybe that's not necessary. 

Of course, CompositeBase is a horrible name, but nothing that encompasses both Composite and Selector has come to my mind yet.  Maybe GeneratorListOperator?  Also seems horrible.