Add support for non-sequntial grain ids

[joelvbernier]

FOr generality, we should not for the grain ids to be sequential, which is tacitly enforced now. This will enable editing of indexing/fitting results without breaking the workflow. Also could be very useful for stitching, removing the need to renumber (@darrencpagan).

Required for HEXRD/hexrdgui#1259

[gustafsonsven]

@psavery This is likely a functionality that could be added alongside a grain ID mask when fitting.