Check for overlap in retained physical groups

[gpeairs]

When rendering to SolidModel, it is possible to create models with overlapping surface or volume entities. This results in either physically overlapping elements, or in elements with two attributes. These can cause errors in gmsh, or downstream in Palace, and should be easily checked for. Loop over retained physical groups, and check that the intersection of any d-group is a d-1 object or the empty set. This should provide faster user feedback before the more intensive meshing or simulation steps.

[gpeairs]

#64 implements a method to check overlaps in a SolidModel. To close this issue I think we want to add it to render! and have it throw a warning (and let the strict keyword decide whether to turn that into an error or not).