Make meshes respect materials priority

[Paul King]

When modelling complex foundations or tunnels, and in particular when foundations and other items that penetrate the ground are contained within modules, it is unnecessarily painful that we must rely on SEO to cut ground meshes.

Every time a module updates (which can be many times per day), any SEO it was connected to in the master/host file is lost and the user must manually return,select all relevant modules and cut the ground again to make cross sections correct.

A great deal of work and rework would be avoided if meshes simply behaved consistently with other element types like walls and slabs.

Note: For hollow spaces underground, user can simply fill voids with an airspace material to ensure ground mesh material does not occupy them