I'm getting much better accuracy sticking with my original
gravity simulator, just with smaller time increments, than
I was with the Runge-Kutta 4 method. The PlutoStateTable
gets a bit larger (51 state vectors instead of 41), but the
accuracy is so much higher.
Removed the Runge-Kutta code because I won't be going back to it.
Reworked the Pluto gravity sim constants so they are defined
in one place: a new header file gravsim/pluto_gravsim.h.
Then the code generator writes the #defines to the C code, instead
of having two independent versions of the same constants.
I will continue down the road of having a single-source-of-truth
for these constants across all 4 supported languages.
Also, confusingly, I had one constant called PLUTO_DT in codegen.c
that was called PLUTO_TIME_STEP in astronomy.c. Also, astronomy.c
had a different constant PLUTO_DT that didn't mean the same thing.
I reworked the naming to be consistent in all places.
I already had a TopPosition() function that knows how to calculate
exact equatorial coordinates, so I eliminated the redundant logic
from gravsim_test.c
