XYZTranslator converts to/from dirs of xyz trajs.

The .nc file can hold all the trajectories in one file; XYZs use directories
of files.
Polished XYZTranslator and its tests.

Reorganized the code, filled in some docs, made comments clear, improved the
tests significantly. Basically, this is done.
Merge pull request #7 from dwhswenson/xyz_iss1

Conversion utils for netCDF<=>XYZ