Wrap coordinates into fractional coordinates

[mattwthompson]

Some formats write coordinates into fractional coordinates, i.e. positions in each dimension are scaled by 1/L into a unitless fractional value. I think there should be a function - probably private - that does this wrapping.

An alternative approach to add this functionality would be to allow mb.Lattice to optionally populate into fractional coordinates. I am opposed to this, however, because the returned compound would have positions in unitless values, not nm.