creates the invert_correct_parallax function, which can take a coordinates in EBD space and returns the position in the original "uncorrected" icrs reference frame.
also fixes an issue where correct_parallax would output as something other than a SkyCoord.
also changes some of the naming around the different "distance" variables (now heliocentric_distance and geocentric_distance, respectively) to improve readability.
resolves #557 , resolves #558
invert_correct_parallaxfunction, which can take a coordinates in EBD space and returns the position in the original "uncorrected" icrs reference frame.correct_parallaxwould output as something other than aSkyCoord.heliocentric_distanceandgeocentric_distance, respectively) to improve readability.