Update MeshManager::get_surface_mesh() to use MBDirectAccess

[Waqar-ukaea]

PR #100 added a new method to the MeshManager::get_surface_mesh() which is used to return the double precision vertices and indices of all of the elements in a MOAB mesh. The reason I set up that PR was to make it more convenient for me to get the information required by GPRT to setup buffers for the double precision geometry in GPRT.

However, at the time @pshriwise suggested #100 (comment):

Down the road, I think accessing this info from the MBDirectAccess class will be more efficient as the triangle/element connectivity maps to the global ID space of the triangles naturally there.

I'm opening this issue to keep track of the need to refactor MeshManager::get_surface_mesh() to make use of the MBDirectAccess class.