also add preliminary script to generate dependency list.
we should use this for compiling in the future.

NSVEparticles test works.

by default a new field object (pressure) is generated every time the
acceleraation is computed in vorticity_equation. I now allow for an
externally allocated pressure field to be used, thus avoiding the
creation of a new field object every niter_part.

basic stats can now be read even in the absence of the main simname.h5
file, using just the cache file.

I'm not sure how much sense this makes, but I am now explicitly dividing
the computed spectrum by dk, such that the integral of E(k)dk is the
total energy.