Introduce a class Xlator for easier parsing and displaying

The complex multi-dimensional 'levels' list has been replaced by
instances of Xlator. Each object has a .parent and .subvolumes attribute
that can be used to traverse the tree (or xlator stack).

This simpifies the algorithm to display the tree and should improve
maintainability of lsgvt.
Add some test .vol files

To use these .vol files for testing, use the following commands:

$ ./lsgvt --path tests
$ ./lsgvt --path tests distrep2x2
Complain if Unicode is resulting in an error