Add regiondata

[jfoster17]

Add RegionData and ExtendedComponent Types

Description

This is a replacement for #2373 which provides just the glue-core pieces of that PR and some additional improvements. As a standalone piece it doesn't do very much, but it provides the underlying architecture so that Viewers can be extended to deal with regions-as-data.

The pieces here are:

• An ExtendedComponent(Component) which takes a list of Shapely geometries and a list of ComponentIDs describing the center of these geometries.
• A RegionData(Data) object which has one (and only one) ExtendedComponent and is the general class to use for any regions-as-data.

Some additions over #2373 are that this no longer requires us to allow_pickle in np.load and it properly round-trips through session files, albeit with a bit of a hack to get the ComponentIDs to be all correct.

Closes #2351 , #2086 , #1686

[jfoster17]

@astrofrog -- I think this is ready now. I would love to get it merged in before opening the PRs for adding RegionData to the viewers.