Modify wallpaper_group.Symmetry and the implementation of wallpaper_group.GroupElement

Modify the implementations to be compatible with #1 (closed).

wallpaper_group.Symmetry needs:

• Need to verify that the translations specified on __init__ is compatible with the declared point group

wallpaper_group.GroupElement needs:

• an r attribute that is a triple (d, t_p, R) where:
  • R is an orthogonal integer matrix defined in the basis of translation vectors
  • t_p is an integer vector in the basis of translation vectors divided by d