Update particles output to limite the number of processes involved -- manage a minimum chunk size -- write a multiple of a chunk

also fix a typo in the vorticity equation wrapper, but the code fails
anyway, so that's not a big deal.
I still need to generate initial conditions.

I doubt that it does anything useful, but it compiles.

found
https://seasonofcode.com/posts/how-to-add-custom-build-steps-and-commands-to-setuppy.html
and used it to clean up the compiling step.

USE_TIMINGOUTPUT should be controlled from setup.py.
In this commit, I'm trying something elegant that's supposed to work,
but the "build" command actually fails for some reason.
Hence the commit, and I need to fix it in future commits.

the idea is to modify the fluid_solver class into something that uses
the field class, to the point where if we want to change the backend to
something other than plain FFTW, the new solver class will not need to
be modified in any way.

the "vorticity_equation" class included in this commit is not
functional, it is only a checkpoint.

however, the error and output files are not in the correct location...

at this point it makes sense, since version 2 is a lot closer now.
and version 2 will work with different input files...

code compiles, but doesn't do anything yet...

the code runs through the first iteration, so now I just need to fix the
redistribution of particles. oh, and the I/O...

this way, rFFTW_interpolator no longer needs to include interpolator.
which will help with splitting the library.

I'm not sure this is the proper way to do things, but I need tox to
work, so I don't care anymore.