Replace omega and omega2 by Constants to avoid recompilation.

To avoid recompilation of the code for different problem sizes, make omega and omega2 firedrake constants wherever possible. On ARCHER there is an issue with recompilation on larger core counts, so weak scaling runs will crash as the grid resolution increases on larger core counts. This workaround should ensure that the kernels only need to be compiled on smaller core counts, and are the subsequently loaded on the larger problems.
Carry out warmup run before timed solve.

To avoid measuring overheads from compilation and loading compiled kernels from disk, carry out a warmup run before the actual solve and reset timers afterwards.
Swap order in mixed function space.

To be able to do solve the Schur complement system for the pressure (instead of velocity, as was done before), swap the order of pressure and velocity in the mixed function space.
New section for general parameter.

Some more general parameters are set in the new section 'General' in the parameter file.
Updated template parameter files for job submission.
Merge branch 'master' into warmup_runs
Runtime parameter for doing warmup run.

Whether a warmup solve is carried out before the actual solve is now chosen via the parameter General->warmup_run in the parameter file.
Merge branch 'make_omega2_constant'
Use diagonal of A_{00} in Schur complement system S.

By using the diagonal of A_{00} to construct an approximate Schur complement S=A_{11} - A_{10}*inv(diag(A_{00}))*A_{01} it is possible to use the AMG preconditioner for the Schur complement system. Otherwise, only A_{11} is used in the preconditioner for S.
Merge branch 'petsc_split_preconditioner'