Add versioned dtd file
After discussions with with @boudreau, an alternative to !146 (closed) which avoids the need for a patch branch.
Instead we introduce versioned .dtd files, that can then be picked up instead of the latest version through changes on the athena side. The naming scheme for this is certainly open for suggestions.
The general idea would be that we establish a scheme in the clob generation for the geometry tags that allows a stable version to be specified, while for development and testing the head version with the latest changes simply named geomodel.dtd can be used. Any time geomodel.dtd has changed since the last tagged version of geomodel, a new file will be added to the data directory, with the same content as the previous tag's head version, incrementing the version number in the filename.