up to now, xmapping was using Trellis_Util::mVector and Trellis_Util::mTensor2.
    This commit change that to further reduce it's dependencies to Trellis, and replace them with xtensor::xVector<> and xtensor::xTensor2<>.
    implyed changes in xmapping :
    - FEM/src/xMapping.h : do not include mTensor2.h and mVector.h any more.
    - xMapping/test/testxLagrangeMapping/main.cc : a binary operator - for mTensor2 was implemented to facilitate the tests. This operator already exist for xTensor2<> and is removed from this main.

    Implyed or other changes outside of xmapping :
    - xCut/xLegacySimpleCut/src/xRefCutToAOMD.h was including xMapping.h. It is useless at current stage and this include is removed.

    - xFEM/src/xGeomElem.cc : some implemetation are simplifyed since we do not need to convert some xVector<> to mVector or xTensor2<> to mTensor2 when calling xMapping members functions.

    - xTLS/src/xRefCutToCGAL.h was including xMapping.h when it just needed a forward declaration of xMapping. This is fixed.