Read the groups after both the nodes and elements are read.

Need to add some checking to make sure that the groups dataset is not
encountered before the elements dataset...

Remember to 'adjust' the stream position.  This should go away once we
start reading the file in a single pass.
Do some error checking of the boundary elements listed in the groups section.
Inserting the elements which provide BCs into a map.
UNV reader now properly assigns BCs.