tfluct as input to solver, following rp

This PR

- adds the ability to provide a _tfluct solver_ to _SharpClaw_ directly in the same fashion as Riemann solvers are passed. When `solver.tfluct_solver=True`, a fortran python-wrapped routine can be given through `import _name-of-tfluct-solver_`, `solver.tfluct=_name-of-tfluct-solver_` 
- solver follows API convention of Riemann solvers
- works in n-d
- euler_1d has been modified to provide an example of usage

I tried to changed the least I could from the original code. @ketch I agree, gamma1 could be calculated internally. We can do the change, but it is out of the scope of this PR. I have added it to the issues list.