Add VR hand controller support for ISOLDE mouse modes

[Tom Goddard]

Add virtual reality hand controller support for ISOLDE mouse modes. This is straightforward and will allow exploring the use of ISOLDE in VR. Create a patch against current ISOLDE to give to Tristan to incorporate the new code.

I already added some trial code for tugging a single atom in ISOLDE and it worked without trouble.

Would also be interesting to allow a mode that allows rotating a residue or part of a residue by rotating the hand controller. Might apply the rotation to atoms in a side chain distal from the clicked atom for easy twisting of side chains.