Build the lumped finite element mass matrix for a diffusion problem.
The equation taken into account is:
delta * zeta * u = f
where delta is an element-wise constant scalar function, while zeta is a piecewise linear conforming scalar function.
See also: bim1a_rhs, bim1a_advection_diffusion, bim1a_laplacian, bim2a_reaction, bim3a_reaction.
Package: bim