Latex wasn't working because the README contains a link to an image that
cannot be converted. So I asked that the image only appears in html.
There were also some strange problems with the markdown math syntax, so
I switched to full LaTeX which seems to be working.

when HDF5 is compiled/installed via cmake, it sometimes installs a
config file that can mess up the exported turtle cmake config.
I.e. libturtle.a compiles/links fine, but executables then fail to
compile because cmake can't find HDF5.
injected a workaround here.

Feature/collisions

See merge request clalescu/turtle!24

these were here for testing purposes, they're not meant to be in the
main repository.

applies same tweaks that were needed for the spherical ghost collision
counter.

for some reason it only works like this on one machine

cmake config can no toggle the use of "NDEBUG" or "USE_TIMINGOUTPUT"
compiler flags, hence source files and/or .bashrc no longer need to be changed
themselves.
I'm not sure the mechanism survives very nicely for executable
compilation, but it's better to have it and figure that out later.

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

change actually made by chichi on Tobias's computer.
blame him.