improvements to GeoModelXML.

first set of improvements to GeoModelXML. Added EllipticalTube (note: no visualisation yet) and TwistedTrapezoid. Corresponding changes in GmxUtil (.h and .cxx) and in geomodeldtd. A spurious copy of geomodel.dtd was removed.

Changes to be committed:

modified:   GeoModelTools/GeoModelXML/GeoModelXml/GeoModelXml/GmxUtil.h
new file:   GeoModelTools/GeoModelXML/GeoModelXml/GeoModelXml/shape/MakeEllipticalTube.h
new file:   GeoModelTools/GeoModelXML/GeoModelXml/GeoModelXml/shape/MakeTwistedTrap.h
modified:   GeoModelTools/GeoModelXML/GeoModelXml/data/geomodel.dtd
modified:   GeoModelTools/GeoModelXML/GeoModelXml/src/GmxUtil.cxx
new file:   GeoModelTools/GeoModelXML/GeoModelXml/src/MakeEllipticalTube.cxx
new file:   GeoModelTools/GeoModelXML/GeoModelXml/src/MakeTwistedTrap.cxx
deleted:    GeoModelTools/GeoModelXML/data/geomodel.dtd