More FM methods added

There are few things here I do not like. The path_t template parameter is not necessary. As done in other parts of the code, I think you could type here directly std::vector< std::array<double, grid_t::getNDims()> > * path and this method would be the same for any kind of type. We are doing what is called static polymorphism, so these API changes should not be necessary if a good design has been done.