Add error checking and profiling options to sequential adv-diff demo
Add advection-diffusion sequential regression test
now advection-diffusion test makes use of ufl 0-form to compute L2 norms
Fix initial condition, error check, end time in adv_diff demo
Advection-diffusion demo: Only print error with --print-output
Use structured unit square mesh
Shorten advection-diffusion test to 50 time steps
advection-diffusion test: assemble matrices only once
Advection-diffusion test: only compute analytical solution if used