renamed gravityPotential to potential.  plans to optinally enable generation of gravity potential vs specify one through the config file.  fixed some math errors in the poisson calculation.  now it works right but looks horrible due to an ever-decreasing potential energy and a few other things. changed velocity field display to display the gravitational field for the time being.  might make that optional in the future.  hydrogpuapp no longer holds the slope limiter name, instead the solver codegen pulls it from the lua state upon macro generation.  added variable control of velocity field arrow scalar.  added gravitational constant to script.