Significant performance improvements through loss of generality.

It is hard to think of a use-case where there won't be at least
1 field and velocities.  Therefore, I've coalesced many of the
operations on the velocities and fields.

The transformed positions are not actually used: just the root
which is invariant.
Added Kolmogorov and Taylor length scales back into spectrum.

The missing piece was ((d u_z / dz)_{z = 0})^2, so Grid.add
now extracts (d u_z / dz) for z slices.