This subroutine calculates fluxes and speeds for hydrodynamic case only.
Adiabatic and isothermal equations of state are supported. I also added
new parameters, gamma and csnd to the config module.
We compute dFdx contribution along each direction and update total dU.
Apart from that I've added generation of coordinate variables, like dx,
dxi, etc. in the mesh module. The next step is to implement the HLL
solver.
This new module contains subroutines to calculate state vector update
using an approximate Riemann solver. The initial version contains only
a draft subroutine calculating the state vector increment dU.
