bench_solve-poisson in bench/fem/speedup/cpp is broken

[Johannes Ring]

When running the bench_solve-poisson benchmark in bench/fem/speedup/cpp, the following error occurs:

*** Error:   Unable to access parameter set.
*** Reason:  Parameter set "petsc_lu_solver.preconditioner" not defined.
*** Where:   This error was encountered inside Parameters.cpp.
*** Process: 0

This is not detected in bench_fem-speedup-cpp (which is run on the benchbot) because this script assumes that all benchmarks are executed correctly.

[Prof Garth Wells]

Fix Issue #64.

→ <<cset 7939643ad017>>