Extended elliptic_fem and elliptic_lodm to more general nonlinearities of lower order and of the type F(x,u,grad u). A corresponding class 'LowerOrderTerm' was added to all model problems. The class can be also used for describing linear advection and reaction. MsFEM and HMM are not affected by these changes (i.e. does not yet work with these generalized nonlinearities).