Add support for visualising waters in the viewer

[lohedges]

This shows waters and the GCMC region in the viewer, and adds options for tuning GCMC efficiency.

• Waters in the 3D views: the binding site and mutation views now show waters within 6 Å of the ligand or mutated residues, as ball-and-stick, with a toggle. With GCMC, ghost waters are left out, using the water state saved in the same checkpoint as the coordinates. Waters anywhere in the GCMC region are also shown, along with the region as a wireframe sphere, centred as loch centres it.
• Sampler statistics: the GCMC table now shows accepted per move, highlighted when above 1 since that wastes trials, and acceptance per attempt. A new chart shows the number of waters in the GCMC region at each checkpoint, for one window or all of them. To time it, both runners now log the simulation time with each water count.
• New options: gcmc_batch_size and gcmc_num_attempts were previously fixed at loch's defaults. They can be changed on restart.
• Fix: the depictions no longer fail for an end state with no real atoms, e.g. a ligand decoupled by turning all of its atoms into dummies.