ENH: Add generic storage node for Markups
Created by: vovythevov
Markups did not have a generic storage node that worked for all for types of markups. This new storage nodes aims to fill this need. It is also meant to be easily extensible in potential markup subclasses.
The new generic storage exports markups to JSON for ease of interface with the web, python and other applications.
The existing Markups Fiducial reader is left as-is and is still the default for fiducials.
CC: @jcfr