this shouldn't lead to a big performance increase of symmetrize itself,
but it should enforce thread-local access to field data. to the extent
that it helps...

unused, but I should fix it anyway

control option: allow fine grained control over parameters given to
mpirun/srun on the command line in interactive runs.

comment: commit adds comment in `kspace.cpp` on division by zero code.
clarification about why it happens, list of solutions.

also, adds new Hermitian_reflect method

problem: consistency check fails with relatively large errors (4% field
and ~16% velocity at particle positions).
But it may be due to I/O.

further checks are needed...

Feature/sample vorticity

See merge request clalescu/turtle!27

makes it consistent with version from `develop` branch. functionality
remains the same.