These assemblers can be used to assemble lumped matrix. They will both do the same thing, aside from
the fact that xAssemblerLumpedBlockEquInVector will assemble in a vector containing the diagonal coeffcients
of the lumped matrix (so it will be used with a bilinear form to assemble in a vector).

These assemblers take each block of the assembled matrix, compute the average and assemble it on the corresponding diagonal coefficients.

For instance, the lumped version of the following matrix

    [a b 0 0]
    [c d 0 0]
    [0 0 e f]
    [0 0 g h]

would be

    [m1 0  0  0]
    [0  m1 0  0]
    [0  0  m2 0]
    [0  0  0 m2]

where m1 = (a+b+c+d)/4 and  m2 = (e+f+g+h)/4

They are not supposed to be used with vectors, so (for now) the assemble_vector gives a throw.