check for multiple files being read simultaneously, and set version to the highest of them

In some cases when reading in XML files for the geometry, two files get combined and read in as one. In this case, the version retrieved from the DB should be the highest of the two, to avoid inconsistencies with the schema versioning.

A warning is printed to make people aware of what is happening, which can be avoided by setting the versions of the two files to be the same in the DB tag, to avoid conflicts.

NB this warning may appear for older geometries if this was not respected (although for those cases it should not have ill effects)