implement Modify and Replace for DSC objects

[LukeCEL]

Is your feature request related to a problem? Please describe. In many add-ons, existing objects are modified using Modify or Replace directives, but this cannot be done for DSC objects (galaxies, globulars, nebulae, and open clusters).

Describe the solution you'd like The Modify and Replace directives should be implemented for DSC objects, in the same way they are for STC and SSC objects.

For example, this DSC code would modify the parameters of WLM:

Modify Galaxy "WLM"
{
	Type "E5" # to show ellipticity of object
	Radius 10040 # longer axis
	InfoURL "http://simbad.u-strasbg.fr/sim-id.pl?Ident=WLM Galaxy"
}

Describe alternatives you've considered In order for the existing objects to be modified, the .dsc files have to be edited directly, but this complicates installing instructions for add-ons and isn't the cleanest solution.

[SevenSpheres]

Already mentioned somewhere I think, but best to have its own issue