The fields currently support a Point source and a two-dimensional
Plane source, but there is no way to create a field that pulls
toward or from a line in space other than using the Texture field
type. This adds a new simple shape option to do that.
The line is aligned along the Z axis so that it works meaningfully
with the Tube and Cone falloff modes.
Reviewers: brecht, mont29, LucaRood
Differential Revision: https://developer.blender.org/D3721
969cbed49b