explore Crystal unit cell transformations

[dkriegner]

This developed out of issue #97

• [x] transform a Crystals unit cell with a transformation matrix https://www.cryst.ehu.es/cgi-bin/cryst/programs/nph-doc-trmat
• [x] allow for choosing different origins of the unit cell
• [ ] likely its then easy to also allow materials by just giving the generators of the symmetry operations

This likely needs also to allow for more flexible unit cell (Base) definitions.

[dkriegner]

the first two points are effectively implemented in 03073e38ce20f42143c09d6f93cf28a976660256

[dkriegner]

The material definition from generators seems to not fit at all to the current way SGLattice is implemented, which heavily relies on knowing the space group and related tabulated properties.

While all this could be re-implemented only using the generators it would be less efficient. If this is a feature somebody needs and he/she can explain why the benefits outweigh the disadvantages a new issue should be reported and one might add this functionality.

Likely one would need to derive a new class from SGLattice which re-implements the problematic methods. The derived class could then serve equally in the definition of a Crystal object.