Crystal improvement ideas

[dkriegner]

Since materials in principle know a lot about their own symmetry they could provide some helper functions to accomplish regular tasks:

• get allowed peaks/hkl: instead of brute force iterating over all integer indices [1,2] see http://pd.chem.ucl.ac.uk/pdnn/symm4/hklcond.htm or http://xrayweb.chem.ou.edu/notes/symmetry.html
• transform its own unit cell with a transformation matrix https://www.cryst.ehu.es/cgi-bin/cryst/programs/nph-doc-trmat see issue #99
• and likely much more

[1] https://github.com/dkriegner/xrayutilities/blob/a2b419fdd2f57e8767bd806a44fa21ebfa101bad/lib/xrayutilities/materials/plot.py#L102 [2] https://github.com/dkriegner/xrayutilities/blob/171ab935204a982124675aec6a5cffc41115402b/lib/xrayutilities/simpack/powder.py#L2183