Automatically set solver.fwave based on values in Riemann repository.

[ketch]

Also check if it's set when checking solver validity.

This addresses the first part of #737 and must be used in combination with clawpack/riemann#181, which adds the relevant values to the riemann repo.

[coveralls]

coverage: 26.759% (-19.8%) from 46.556%
when pulling 9266b53 on set_fwave
into 37658ee on master.

[mandli]

The advection_reaction_2d example is failing for me (as is the shallow sphere example but that I somewhat expect). It is complaining about solver.fwave not being set.

Log from pytest.

[ketch]

The advection_reaction_2d example is failing

Thanks. Yes, now the solver.fwave must be set manually if you're using a Riemann solver not from the riemann repository. In that sense I guess this is a breaking change but it is quite small. We should probably mention it prominently in the next release notes.

I fixed that example by adding a line to set the value correctly. I didn't find any other examples that are affected by this.