Add Lua-controlled forcefields

[yogwoggf]

Context

Fairly simple, there's lots to gain from forcefields such as: fluid grabbing, "milk" monster, black holes, etc.

Scope

• Add forcefields to the scene as object oriented structures
• Expose them in Lua

Acceptance Criteria

• Forcefields can be created in Lua and destroyed

Other

Simulation will gain a new public API member for forcefields

[yogwoggf]

Nearly ready in branch 99-forcefields. We went with GellyX exposing forcefields as SENTs and Gelly exposing forcefields as object handles.

It's a little gross, but gelly-gmod is in desperate need of a touchup