Consistent energy management

Earlier on, energy consumption was calculated before calculating the
resulting linear/radial velocities. This was arbitrary, because linear
and radial impulses have a very different cost. Now I apply the
multiplication factors before updating energy.
Update backlog
Fix bugs in center of mass calculation

First of all, it was calculated erroneously for each organ. To boot,
there was a hidden cast to int in the main calculation that might also
have resulted in tiny errors. The first problem was huge: essentially,
the center of mass was completely wrong (shifted much closer to the head
than it should have been - in large creatures, this resulted in a big
error).
Move all movement-related methods from Narjillo to Body
Add center of mass view for debugging
Fix bug in translation force calculation

I multiplied by the mass instead of dividing by it. Well, this explains
in part the huge difference between linear and rotation multiplication
factors. It also explains why Narjillos were getting huge in long
experiments: the bigger they were, the more they moved around. Whoops.
:)
Fix bug in calculation of translational energy

The energy was calculated based on the naked mass, instead of the
corrected mass.
Add one angle normalization in Head#move()
Make Narjillos' energy proportional to their mass
Fix bug in linear momentum

I was considering any movement of each body segment in any direction.
Instead, only the normal component of the movement should count. A body
segment doesn't generate any push if it slides along its axis.

This code also contains a first stab at implementing a bonus for being
fast, in the form of a viscosity factor (on the whole body only).