pnfft: fix init_x_3d_adv

would always produce x > x_max
pnfft: tests: use pnfft_init_x_3d{,_adv}
pnfft: allow tests to be run from any directory
pnfft: add function for debug printing
pnfft: dont print debug spam from loops
pnfft: store fourier coefficients as R*
pnfft: fixes for C -> R
pfft: add apr_real_3d for printing real 3d-arrays
pnfft: add apr_real_3d for printing real 3d-arrays
pnfft: add r2c test
pnfft: add test to check r2c/c2r error
pnfft: add defines for r2c/c2r transform
pnfft: c2r/r2c
pnfft: add C2R/R2C to pnfft_flags in init
pnfft: store pfft input as R*
pnfft: actually make r2c/c2r transforms work
pnfft: make c2r work
pnfft: make c2r work
pnfft: WIP (r2c)
[WARNING] This branch will be rebased (enable debug)

at the moment, r2c/c2r transforms don't work yet,
but at least c2c transforms aren't broken