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

My point was that you are already in the flux1 function but now that I look at the SharpClaw code, flux1 is called from flux2 (which is not the case in classic, there flux1 does not exist at all).  I see why you are doing this now.