Retire MPD

[N. W. Perry]

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.

And of course parts contain other parts, but they do it by referencing other files according to an expected file structure. I suppose if submodels were referenced the same way without needing to inline them, that would obviate the need for the mpd format.