Clean unused declaration.
Use direct access to numpy arrays for Atom attributes.

Direct copy of the data should be faster than creating a
numpy-array-view and using array assignment.  This speeds up
writing to Atom.xyz_cartn and Atom.uij_cartn members.
Improve conversion from diffpy.Structure.

Return instance of AtomicStructureAdapter or PeriodicStructureAdapter
when diffpy.Structure has no pdffit metadata.  Otherwise return derived
classes DiffPyStructureAtomicAdapter or DiffPyStructurePeriodicAdapter.

Avoid use of nosymmetry wrapper when Structure is on Cartesian lattice.
Require exact Cartesian lattice for AtomicStructureAdapter.

Allowing small difference from Cartesian base would invalidate
calculation of numerical derivatives when lattice parameters are
close to Cartesian.

Solution: Require exact lattice parameters.