Add Lattice attribute isotropicunit.

isotropicunit is an isotropic ADP matrix in the given coordinate system.
Add method isanisotropic to the Lattice class.

This checks whether a 3x3 ADP matrix is anisotropic.
Optimize Atom ADP matrix and anisotropy.

- convert Atom.anisotropy to a normal attribute
- adjust U values when anisotropy is flipped.
- do not change anisotropy flag on U assignment
- store Uiso in U[0, 0] for isotropic ADPs