unify alias for complex-type from fftw_interface

See merge request !146

use type alias for complex type in kspace

See merge request !145

In case of any changes of the usage of the complex type from within
the kspace and/or changes of the fftw_interface where the alias is
referring to, now only a single line has to be adjusted.

adds error message

See merge request !144

fixes the generation of slurm scripts

See merge request !142

Remove superfluous move-statements.

See merge request !143

These were superfluous and inhibiting copy elision.

the code should now work for clusters with arbitrary numbers of cpus per node.

Doc/fix docstrings

See merge request !141

fix get value in docs

See merge request !140

fixes variable name

See merge request !139

Bugfix/nse initial condition

See merge request !138

adds text on how to launch jobs

See merge request !137

changes location of "turtle_config.hpp"

See merge request !136

this is such that no conflicts arise when using multiple build
directories.

gets rid of cmake FFTW warning

See merge request !135

adds mechanism for control of initial condition behavior

See merge request !134

refactor initial condition

See merge request !133