STY : cleaned up Import.xtomo_reader

 - explicitly use slices via `slice` object rather than implicitly
   (via [start:stop:step])
   - this removes the need for setting defaults to the size
 - added @staticmethod to methods that are static.

 - Import should really be a module, not a class.
STY : more aggressive refactoring

 - should still be backwards compatible
Merge pull request #3 from tacaswell/cleanup

STY : cleaned up Import.xtomo_reader