layup.constants
Physical constants for layup, defined once.
Before this module these values were re-declared in several places
(orbitfit.py, predict.py, iod.py,
utilities/data_processing_utilities.py). The duplicates happened to agree
numerically, but independent definitions are a drift hazard – e.g.
SPEED_OF_LIGHT was written two different ways (via metres in some modules,
via kilometres in another) that only coincidentally produced the same value.
Importing everything from here gives a single, citable source of truth.
Units follow layup’s internal convention: distances in astronomical units (au), times in days, so GM values are in au^3/day^2 and the speed of light is in au/day.