Minor refactorings in Narjillo
More small refactorings
Remove unnecessary ForceField parameter from angles calculation

First step towards separating calculation of angles from
position/rotation update
Separate update of angles from update of position

This was a crucial commit because it seems to remove swarmers. Detailed
comments in one of the next few commits.
Move angle reference point to head
Remove constraints on lateral movement

While rewriting the physics code, I apparently managed to eliminate
swarmers much earlier than I hoped. :) I was assuming that removing
swarmers would have required an entirely new physics engine, to avoid
creatures with a small head that barely moves, and a big tail that
oscillates wildly. Actually, the problem with swarmers seemed to be
related to the way I managed angles. Creatures still have an
"impossible" angle setup (the "tail wiggling the dog" effect), but this
doesn't seem to be a huge problem. I can probably live with it.