Refactoring crystallography base classes.
Refactoring crystallography base classes.

Refactoring the `LatticeBase`, `ReciprocalLatticeBase`,
and `StructureBase` classes in the
`sknano.core.crystallography` subpackage.

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