More flexible generation of crystal packing

[ghutchis]

It's clear that it would be useful to have more options in terms of display and "filling" of unit cells, supercells, etc. (Right now, supercell generation is limited to integers.)

For example, selecting or generating atoms from 0.0 to 0.99 or 1.5 in reciprocal space, or:

https://discuss.avogadro.cc/t/cif-handling-should-be-optimized-i/7287/22

[ ] packing of a unit cell with
  [ ] atoms which fit
  [ ] atoms of molecules whose centroids fit
  [ ] atoms of molecules where any atoms fit
  [ ] atoms of molecules where all atoms fit
[ ] display of the asymmetric unit