The pde tests fail sometimes.

Apparently, this can only be reproduced on some machines.
On neon (72C) one can reproduce the error (eventually) by running:
while ./pde/tests/test_pde_boost --run=\*/testOperationMatrixLTwoDotExplicitLinear;do;done;

I believe this bug is related to #45 as we have some duplicated source code here and this issue was never fixed.