Up to now, xmapping was only containing the header xMapping/src/xMappingBuilderHolder.h, letting construct mapping from trellis.
This commit changes it to a real library, in order to push there all the mapping need of eXlibris. Now mapping can be used with only name and include from xmapping, with no more direct include from trellis.

Rationnal : paving the way to less dependecies in Trellis.

For now, no modification are needed in client code. In next commits, the
changes will be implemented so that only include from xmapping will be
used in clients of mapping codes. after that, new implementation of
mapping could appear with no mention of trellis.

new file xMapping.h : define our own xMapping type, for now just a new name
for Trellis_Util::Mapping.

file xMappingBuilderHolder.h : some of the implementation have been
moved to the new file xMappingBulderHolder.cc.
new member function added to xMappingBuilderHolder : buildMapping. This
simplify the creation of mapping in client code.

new file xMappingBulderHolder.cc : contain some of the implementations.

xMapping/CMakeLists.txt now builds a library.
xUtil/cmakeUtil/FindxFEM.cmake : now xmapping library is requiered.

updated and new files are clang-formated.