xmapping now have it's own enum type xReferenceElementType defined in
xReferenceElement. It is there to replace Trellis mType inside xMapping.
The xMappingBuilder are moved to xfem. Rational : The builder work on
mEntity that are not supposed to be known by mapping.
This being done the only dependencie left in xMapping is the dependecies to
xtensor.

a function is added in xAOMDEntityUtil.h to get the xReferenceElement from
the mEntity, as well as conversion fonction from xReferenceElement to
mType. In the current version of the library, this function is only used
in xfiles to create  a Trellis_Util::GausIntegrator out of a xMapping.
Therefore these conversion function might become useless when we make
our own Trellis free Integrator.

Most of the other change in this commit are consequencies of the above.
Small other minor change are listed below :

-xfiles_dependence.dot has been updated with xmapping dependencies.
-delete mapping added in xElement::xyz2uvw...