Move comment to more relevant location
print_element_residuals option in adjoints_ex5

This helped me debug the fem_system_rates change; might as well leave
it in as an example for others.
mass_residual should now *subtract* terms

This makes it compatible with the new EulerSolver code and runs faster
than changing the solver would.  It's an API-breaking change, but in
an API that I was already in the middle of breaking.
Fix new Euler2Solver evaluations

When refactoring I forgot the theta,1-theta scalings