DiffContext::elem_solution_rate, accessors

Also refactor the FEMContext accessors to reduce some code
near-duplication.

At this stage we're not actually using elem_solution_rate; that
semantics breakage will come later.
Initialize solution_rate,instantiate interior_rate
Use solution rates now in mass_residual

This is an API-breaking change for many DiffSystem-based codes
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