Add create methods on DAGModel class

[paulromano]

This PR builds on #13 to providecreate_group, create_volume and create_surface methods on the DAGModel class that simply delegate to the underlying DAGSet.create and Group.create classmethods.

[gonuke]

I made a few docstring suggestions to remind users that the new groups are created empty, and that the volumes and surfaces are really sets.

Also, another test suggestion, but not 100% convinced that it adds to the coverage.

[gonuke]

LGTM - I'm ready to see this merged

[pshriwise]

Agreed @gonuke. Thanks @paulromano!