[xFEM] Added xAssemblerLumpedBlockEqu and xAssemblerLumpedBlockEquInVector.
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.
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