Convergence test

[krober10nd]

At a minimum we need to demonstrate and have a unit test which verifies all implemented numerical methods are converging at the correct spatial order

for very high order methods, the cfl condition will need to be much more conservative than the stability criteria in order to satisfy the asymptotic regime criteria.

We may also think about higher order timestepping methods

[jaimesouza]

I agree. I am gonna work on that this week.