Turn on_groundinto a Python utility

[dokempf]

So far, this has been a platform setting. But the Python programming language makes this much better as a utility. I propose the following interface:

class PlatformSettings:
	def force_on_ground(self, scene: Scene):
		# Calculate how to get from current (x, y) to a scenepart
		# labelled as ground by following the direction vector.
		self.z = ...

An alternative API that requires redundant use of xand ywould e.g. be:

from helios.util import force_on_ground

settings = helios.PlatformSettings(
  x=x,
  y=y,
  z=force_on_ground(scene, x, y)
)