Cherry-picked two commits from GeoModel master

See merge request !93

Introduced two new methods for GeoVFullPhysVol for accessing cached absolute positions

See merge request !79

GeoXF.cxx makes heavy use of Eigen.  In debug builds it ends up being
much, much slower than in optimized builds, to the point where where it
makes it difficult to run things in dbg.  Tweak the build so that we compile
GeoXF.cxx with optimization, even in dbg builds.  We also need to use the
flatten attribute on the functions that use Eigen, so ensure that the Eigen
operations are completely inlined.

This was motivated by the fact that numerous tests are timing out in the dbg
build.  This change alone reduced the time required by one test from ~600s
to ~400s.

Fixing bug in the GDMLtoGM materialHandler.h. FractionList must be cleared for new materials (D.Csper)

Suppressed the "HowTo section".  Will ask Riccardo to migrate the dump-geo to GeoModelATLAS documentation because this is ATLAS-specific and Athena-based. 

Comment out paragraphs which are totally empty and probably redundant with the original documentation.

Docs test

Closes #12

See merge request !50

Documentation fullsimlight

See merge request !48

Fix typo in spelling the parallelepiped and moved section of GeoTrd where it belongs in alphabetical order

This reverts merge request !49

use built-in Xerces, not from apt-get

See merge request !49

Documentation: added two sections to the GeoModel Kernel Overview

See merge request !45