Include SNR candidates into DSO catalog

[alex-w]

Currently we have supernova remnants (Green's catalog of galactic SNRs), but not candidates to SNRs

See also discussions in #4709 and in #4634

[MaraJadeLives]

IIRC, SNR candidates are supposed to display in Stellarium.

Green has a catalog of candidate SNR remnants in galactic coordinates only, no RA/Dec. You'll need to use a Galactic to RA/Dec converter to get RA/Dec, I like this one.

There are also a few SNR candidates that have been proposed in relatively recent papers that never made it into his catalog, as far as I know, that also use the GX.X[+/-] X.X format.