Updating the `NanoStructureBase` class.
Updating the `NanoStructureBase` class.

* Moving methods from `BasisMixin` class to `NanoStructureBase`.
* Updating the `element1`/`element2` setter methods to
  call the `CrystalCell.update_basis` method.

androomerrill/scikit-nano,androomerrill/scikit-nano,scikit-nano/scikit-nano,scikit-nano/scikit-nano