Add ROI data type

[tyarkoni]

To facilitate ROI-level meta-analyses, it probably makes sense to add an 'ROI' data type under contrasts (in addition to 'coordinates', 'images', 'connmat', etc.). Alternatively, maybe 'parcellation' is more general. Basically, we need to allow for a list of (possibly overlapping) multi-voxel parcels that have an associated value (or set of value), definition of the parcel (e.g., image mask), method of extraction, etc.

[tyarkoni]

Closing this—following discussion, we established that we can extend the Point (formerly Coordinate) object to easily integrate this functionality.