xMapping now include xTensor/src/xBoundingBox.

The following virtual member function of xMapping have been added.
They all have default implementation in terms of the one existing before.
They mostly simplify code when using them, and do not need to be implemented i
in derived class (but could for optimization purpose)

 virtual xtensor::xPoint eval(const xtensor::xPoint &uvw) const;
 virtual tensor deval(const xtensor::xPoint &uvw) const;
 virtual bool invert(const xtensor::xPoint &xyz, xtensor::xPoint &uvw) const;
 virtual double jacInverse(const xtensor::xPoint &uvw, tensor &invjac) const;
 virtual double detJac(const xtensor::xPoint &uvw) const;
 virtual bool inReferenceElement(const xtensor::xPoint &uvw) const;
 virtual xtensor::xPoint COG() const;
 virtual double pushBack(const xtensor::xPoint &uvw, size_t vsize, vector3d *vec) const;
 virtual double pushBack(const xtensor::xPoint &uvw, std::vector<vector3d> &vec) const;
 virtual double pushBack(const xtensor::xPoint &uvw, size_t vsize, tensor *tens) const;
 virtual xtensor::xBoundingBox boundingBox() const;

interface change in xMapping :
    virtual bool interiorCheck(const xtensor::xPoint &p, double u, double v, double w) const; changed to virtual bool interiorCheck(const xtensor::xPoint &p, xtensor::xPoint &uvw) const;

 Modified files :
   xMapping/src/xMapping.cc
   xMapping/src/xMapping.h

 and
   xFEM/src/xRegularGrid.cc
   modified to reflect xMapping::interiorCheck interface change