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 `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 think I see what you are saying although now having different signatures for each dimension's implementation (which I think is fine) means you may want to have the stub mimic this (i.e. have stub for `tfluct1`, `tfluct2`, and `tfluct3` which have the correct signatures).  Often times users will copy these stub functions and use them so this could be dangerous otherwise even if you are passing them through to `flux*`.