Wrapper for hyperbolic class and fixing ex18

[justinlaughlin]

https://github.com/mfem/mfem/pull/3598 introduced a new class DGHyperbolicConservationLaws and refactored ex18.cpp and ex18p.cpp.

The new implementation leads to ex18.py failing; the solutions are very similar but different enough to not pass. For example

• differences in time-stepping due to (I believe) changes in GetMaxCharSpeed result in 435 time-steps to reach t=2.0 rather than 437.
• Meshes/solutions were renamed from "vortex-..." to "euler-..."

I'll start branch to wrap the new hyperbolic conservation class and update ex18

[justinlaughlin]

https://github.com/mfem/PyMFEM/tree/fix-ex18

[sshiraiwa]

@justinlaughlin I started from fix-ex18 and rebased it use mfem_47_dev as a base. https://github.com/mfem/PyMFEM/tree/fix-ex18-v2 I added hyperbolic.i for serial and parallel (build passed, not used yet from ex18).

I will take a look at ex18

[sshiraiwa]

Closing this.