Add GLTF export of atomic structure morphs

[Tom Goddard]

The GLTF format supports morph animations which could be used to show a ribbon or atoms morph. The implementation uses displacements of the original mesh positions and normals, so it can't handle different frames having different meshes as would be needed for a molecular surface morph. But ribbons and atoms should keep the exact same number and order of vertices and triangles.

This might be of interest for the NIH 3D archive.

In addition to ribbon or atom vertex and normal displacements for each frame of an atomic model coordinate set the GLTF also include and "animations" key and key frame times relative to a global clock in seconds are given and for each time a set of N weights, one for each displacement set (called a "target").

Probably colors could also be morphed, for example, to highlight interesting residues.