v0.2 - Prune Feature Added

This release constitutes contains the additions:

• The Grain class no longer ad-hocs a polycrystalline array, this is handled by the Polycrystal class which instantiates and interfaces with the ase.Atoms class. This produces a more robust and efficient implementation.
• The pruning of overlapping atoms between grains has been partially addressed (see #1), this is implemented by using the ase.neighborlist class to build and search for atom neighbors of a polycrystal object. An neighboring atom is removed upon identification that it falls below the specified cutoff. At the moment this is done for all atoms within the polycrystal object (i.e., bruteforce). Next release will include a routine for identifying grain-boundary atoms and check distances between atoms in neighboring grains. This should greatly speed-up the pruning process.