Improve Derived Region Usability

[jjnesbitt]

Follow up to #19. These are the main bullet points so far:

• Add styling to highlight the currently selected regions from a dataset when creating derived region (dull un-selected regions from the same dataset)
• Add a color selector option when creating a new derived region (default to random)
• Add dataset intersection/subtraction as a way to create derived regions
  • Initial use case is to intersect a region dataset with a transport dataset, returning a derived region which contains all regions from the initial region dataset that intersect with the transport dataset.
  • Would probably be some kind of "Derived Region Builder" interface, that allowed for selection of which datasets to create from.