Reinstate the PLY parser

[fredludlow]

This seems to work... (TS conversion of the latest PLYLoader from the three.js project and adaptation into the NGL parser infrastructure)

The parser/ply example is active again.

Fixes #1049

[fredludlow]

@ppillot - If you have time / would like to review please do so - if I hear nothing in a week or so I'll merge and try and remember how to release :)

[ppillot]

Tested and works well!
("Representation / Label" is also fixed with this PR)

For the release, npm version minor should do the trick