DiscreteFunctionP0.hpp

This allows standard operations that are already available for basic
scalar types B,N,Z,R and R^d, R^dxd

For instance if a:R, v_d:R^d, M_d: R^dxd and ah:Vh(R), vh_d:Vh(R^d),
Mh_d: Vh(R^dxd), one can write :
a*v_1, M_2*v2, M2*vh_2, a*vh_3, M_2*Mh_2, Mh_2*M_2, a*vh_2, a*Mh_3,...

Invalid constructions are for instance
v_1*a, v_1*M_1, vh_2*M_2, vh_2*Mh_2, a_h * a,...