implement ~Group

[gdementen]

This would return Axis[:].difference(Group).

For example,

>>> a = Axis('a=a0..a4')
>>> ~a['a0,a3']
a['a1', 'a2', 'a4']

[gdementen]

Easy but not possible with current group API (because ~ is already defined). We need #23.

[gdementen]

removed easy label to avoid a new contributor trying to tackle this before #23 is done