converted MapBase to an NDData stand-in.  Corrected all property attributes so that they are correctly read-only.
converted MapBase to an NDData stand-in.  Corrected all property attributes so that they are correctly read-only.

dpshelio/sunpy,Alex-Ian-Hamilton/sunpy,Alex-Ian-Hamilton/sunpy,dpshelio/sunpy,dpshelio/sunpy,Alex-Ian-Hamilton/sunpy