Retire MPD

[Willy Tschager]

In light of this thread:
https://forums.ldraw.org/thread-24589-po...l#pid41128

I'd like to renew my call to retire the MPD extension and to go LDR only for model files. All software that claims to be LDraw compliant should implement the MPD spec and there is functionally not difference between the two. Since it clear that at least some users are confused, I'd like to simplify things.

If I remember correctly there was confusion among the casual user that a ldr file could not only contain dat files, so parts, but also other ldr files called submodels. From an organisational point of view it should be handled like this:

dat - parts
ldr - single model
mpd - multiple models

w.