It uses PyClaw's tests since, to my understanding, an ensemble
of local regression tests for the Riemann repository using
Pytest would require the appropriate argument intents of the
rp* subroutines to be explicitly declared already.
So the Riemann solvers should be modernized first.

Also, the test for shallow water on the sphere is omitted for the moment.

The PyClaw test that uses this RS is skipped in the testing
workflow since it leads to a core dump
(I suspect this has to do with the OS or the system
architecture).

However, this test passes locally for me (before and after the
implicit none changes).

The regression tests in AMRClaw are not being collected by Pytest. They
just have to be renamed. This is a temporary fix to run them from this
repo in a single pytest session.

Update branch, add testing workflow, use implicit none in more Riemann solvers