This reverts commit 9d3f0eae.

Need to do rigorously including implications for gathering data.

Bird's method for a 2D code involves all vector quantities being
actually 2D, and the density (for example) being particles per unit
area (flowfield) or length (surface).  Needs some thought to
generalise.

Saving the full 3D velocity before constraining for tracking, then
restoring afterwards.

than when it is initialised.  This allows messages to be created and added to.