Add support for groups

[Teester]

Currently shex groups are not supported and the grouping is ignored.

Thus the following from E228 will not correctly evaluate:

{
(
pq:P1534 @ +;
pq:P582 xsd:dateTime +;
)* ;
}

This translates as must have 0 or more of the following: 1 or more P1534 and 1 or more of P582.

shape.py evaluates it as must have 1 or more P1534 and 1 or moreP582, which is incorrect.