Retire MPD

[Travis Cobbs]

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.

I agree (as probably would be expected based on my posts above), with the caveat that we need to make sure all LDraw programs still recognize the MPD extension, just not create new files with that extension. So, I don't think "retire the MPD extension" is the best way to word things.