create Body.field property and cdef Children Scene._fields

[ArcRiley]

soy.fields should work like soy.shapes and soy.models; it adds functionality to a Body.

soy.fields.* should not inherit Body. They should pull their position from the Body they're attached to, and either work position-less without an attached body or disabled. They must also be able to exist in the abyss (outside a Scene) for loading/saving.

Instead of adding themselves to soy.scenes.Scene._bodies they should add themselves to a Scene._fields Children. In Scene._loop this should be iterated over for field actions and should do so once per ODE step (moved lower in the source), not once per _loop, for temporal consistency.