checkpoint is somewhat improper because checkpointing functionality was
implemented in vorticity_equation instead of
direct_numerical_simulation, so that would be the next step.

preliminary test works with no obvious problems.

this is not nice at all, but the purpose is to have basic functionality
for testing. It's pretty obvious this will need a custom launcher for
production work.

now code compiles

NSVEparticles test works.

not tested yet.

Code now computes vorticity stats first, and then velocity stats.
This is relied upon in the `NSVEparticles` class later to avoid an FFT.

NSVE (and implicitly NSVEparticles) was using FFTW_MEASURE when creating
the `vorticity_equation` object.

this means i defined a virtual "step" method, as well as "do_stats" and
"write_checkpoint".
I think this makes it even more clear what job is assigned to which
class.

I need to figure out how to define the read_parameters method properly