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.
Unfortunately, that doesn't seem to work. If I do that Python complains:
```python
pattern_type = param.ClassSelector(default=FileImage)
```
TypeError: __init__() takes at least 2 arguments (2 given)

__init__() needs the class you want to select from as argument. I do not know how to pass that from the superclass. If I then use
```python
pattern_type = param.ClassSelector(PatternGenerator,default=FileImage)
```
The is_instance attribute gets overridden with the default, so I have to use
```python
pattern_type=param.ClassSelector(PatternGenerator,default=FileImage,is_instance=False)
```