[XFEM] change API of LOCALVECTOR concept (xAssembler concept)
This commit pass from () to  operator for LOCALVECTOR concept of xAssembler concept. This is opening usage of assemble_vector method to a wider range of container types. LOCALVECTOR may now be a xCSRVector or a std::vector for example. xFemVector having both operator remains functional (maybe () operator should be removed now ?). Some comments are added to try to clarify LOCALVECTOR and LOCALMATRIX concept.
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