Add a new case to the GXClashPointSystem, related to invalid solids (i.e. boolean solids)

A solid is considered not correctly defined, then invalid, when it's not possible to generate points on its surface (mainly happens with boolean solids). In this case gmclash writes an entry in the json report, writing as coordinates the global coordinates of the center of the invalid volume. The field distance is filled with a generic non-valid value: -999 With this merge request we can visualise these points in gmex, under the checkbox "InvalidSolid", color folly.