Include the Voyager Jupiter profiles with extrapolation to lower altitudes.
Converted to python3 with 2to3.
Some additional changes for the transition from python 2 to 3.
Ignore negative electron densities in the input.  This is needed bc some
observations are provided with negative values.
Corrected Voyager profiles for Jupiter.
Solved a bug when tracking with a domain that does not start at (0,0)
Use a 90:10 mixing ratio for the Saturn data