Move TMD reader out of Scene.js

[dannycalleri]

Even if the format is specified inside his own TMD.js file, the reader it's still inside Scene.js's loadModel.
Scene.js should become agnostic to the type of model we are loading.
It could use a switch statement to now exactly which format specifier and reader to load, starting from file extension.

A file TMDReader.js already exists inside src/formats/

See also #1