Generalization for arbitrary initial condition

IC can be specified on command line:

    * Location of initial discontinuity (0 <= x <= 1) : -x0
    * Density left/right                              : -rho1 / -rho2
    * Velocity left/right                             : -u1 / -u2
    * Pressure left/right                             : -p1 / -p2
    * Gamma left/right                                : -g1 / -g2
    * Correction pressure left/right                  : -pc1 / -pc2