CudaClaw : Write step2 and flux2 kernels

[donnaaboise]

These routines compute fluctuations fp, fm, gp, gm and need to be written as a CUDA kernel(s).

• Step2 computes fluctuations by sweeping in x directions and then in y directions. The routine flux2 is called for each horizontal or vertical strip of grid data. The routine flux2 then calls a Riemann solver for each grid interface.

• I think it might make sense to combine step2 and flux2 to a single routine which calls a thread for each cell interface.

• To start, let's just write a version of the flux computation that does the first order spilt update. That is, code that handles everything through about line 124 in cudaclaw5_flux2.f

[donnaaboise]

We have the basic routines in place for this, so will close the issue. Still need to get limiters, second order corrections, and transverse corrections in place, though.