Feature/stokes drag

See merge request clalescu/turtle!26

to simplify debugging of stokes drag code.

`NSVE_Stokes_particles` currently hangs on my computer, with no
error output at all.
I will debug later

create dedicated folder for testing, fix compilation error for
NSVEp_extra_sampling and remove unnecessary NDEBUG definition from
`ornstein_uhlenbeck_process.cpp`

there was an implicit assumption that the file exists before.

List of checkpoint files is now generated explicitly (rather than
listing files that match "checkpoint_*").
This fixes problems when encountering non-checkpoint files that do
contain the "checkpoint" word in their name.

initial condition must be generated even for unlisted solvers.
This commit also changes the name of a variable for style reasons

I think this line got lost in a merge somewhere... Not sure.

we're using clusters that require different slurm configs, hence the
need for fine grained control.

use less iterations, and only run one job.

this test puts 9 million particles on process 0, 1 million on process 2,
and 0 on the other processes (initially).
It's meant to test whether everything is working when some processes
don't have any particles.