Bugfix/ideal ball with chease equilibrium v2

Fix bug where ideal_ball crashed with CHEASE equilibria

Fixes #90

Add warning comments about possible ideal_ball bug

This commit adds comments rather than fixing the bug becuase the use case of ideal_ball with a numerical equilibrium is rare. Therefore, the benefit of having the bug fixed (potentially no benefit) is outweighed by the effort of checking to see if this is a bug, fixing it and testing it. However, if the bug does ever crop up in the future, there are at least some comments to give the fixer a clue as to the likely cause and a possible fix.