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

What's the reason for changing this?  I think it's annoying that we require the user to set gamma1.  I think that we should change the Riemann solvers that still have this so that gamma1 is just computed in all of them and the common block contains only gamma.  @sanromd do you mind making that change in Riemann?