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.