provide documentation for skeleton viewer, allow user to input rotation angles, allow user to take journal-quality figure pdfs

[BenjoCowley]

There needs to be more documentation for the skeleton viewer (or please make it easily accessible---I couldn't find any). What are the keyboard shortcuts to zoom/rotate?

In addition, the user should be able to input the precise coordinates/rotation angles for the skeleton viewer for reproducibility between different sessions.

In addition, users should be able to export the skeleton view as a pdf for figure making (journal-level quality).

#enhancements for skeleton viewer