Expand openmc.lib.dagmc functions into a new DAGUniverse class.

[pshriwise]

Description

The openmc.lib.dagmc module is being added in #3056 to support differentiation of DAGMC cells and materials for depletion calculations using DAGMC geometry. A couple of functions:

• get_dagmc_cell_ids and
• get_dagmc_universe_num_cells

have been added in support of this effort.

It would be good to turn this into a class, openmc.lib.DAGUniverse that supports the same methods as the openmc.lib.Universe class in the future.