Added PatternCoordinator class which returns a set of coordinated pattern generators
Merge branch 'master' of https://github.com/ioam/imagen
Added CompositeBase superclass for composite pattern generators
Rename dims to features, ValueCoordinator to FeatureCoordinator, various variable renames according to the comments in the pull request, improved docs, default values for dataset in PatternCoordinatorImages, improved example for inherent_dims
Consistent naming of attributes
Using underscore for attributes and methods which are not intended to be used by the user
Setting default values for JSON object with .get() method
Updated and extended docstrings
Rename FC_x to XCoordinator etc, _feature_coordinators_to_apply with underscore
Merge pull request #24 from Tobias-Fischer/feature

Added PatternCoordinator class, which returns a set of coordinated pattern generators.  

Also changes the default of imagen.Selector, but we do not expect this to be a problem given how arbitrary the default was before and thus how unlikely it was that anyone is relying on the default.