use type alias for complex type in kspace

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.

@chichi whenever grepping for fftw_interface I was stumbling across all these lenghty type specs so I thought maybe it would be nice to reduce that to a single definition that is easier to maintain in case?

approved this merge request

enabled an automatic merge when all merge checks for bdbd53f2 pass

resolved all threads

merged

mentioned in commit 2f0fc916