Check for overlap in retained physical groups

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.

The issue names SolidModel rendering and retained physical groups but no source files or tests. Start by locating the SolidModel rendering entry point and the code that stores retained physical groups, then inspect existing geometry-intersection checks. Done means invalid overlaps are reported before meshing or simulation, including intersections that are empty or one dimension lower.

Written by the indexing model from the issue text.