Modify the abstract 'Symmetry' and 'GroupElement'

Symmetry

• Should know about its periods
  • We will have to modify the __init__ checks to verify that the periods are compatible with the declared point group
• Remove abelian_normal_subgroup, as it serves no real purpose

GroupElement

• Should know about its translational (t) and point group (r) part.
  • t is always a vector of integers (in the basis of periods) (or None)
  • r is an opaque object, or None