OP2/PyOP2 — pull request #417 by wence- (2014-12-12 12:23:04+00:00)
Title: Always reserve space for (and zero) diagonal matrix entries

Many PETSc solvers require that the diagonal entry of the matrix be
set (even if it is zero). If integrating over a subdomain, our previous
approach of just inspecting the maps occasionally would lead to us
neither allocating, nor zeroing, diagonal entries. So fix that.