Remove use of xAttachableGP in Xfiles. Now a datamanager is added to class to attach Gauss Points data.
    The type xGaussPoints is defined as std::vector<std::pair<xtensor::xPoint, double>> and the datamanager type is datamanager_gauss_points_t (defined in xIntegrationRule.h).
    A static function of xIntegrationRule allows to get the datamanger.

    Example 1:
        agp = new xfem::xGaussPoints;
        xfem::xIntegrationRule::get_gauss_points_datamanager().setData(*e) = *agp;

    Example 2:
        xfem::datamanager_gauss_points_t& _data_GP_= xfem::xIntegrationRule::get_gauss_points_datamanager();
        xfem::xGaussPoints* agp = _data_GP_.getData(*e);