TODO

#pragma GCC optimize ("string"...)

automatic cut-off in PBC - the minimum-image convention

read http://dx.doi.org/10.1107/S0021889800019993

SSE/AVX intrinsics for better performance?

header comments in generated patterns:
 - put more information (from/to/step) in the first line:
   #debyer-pattern reduced-PDF [0 : 6 : 0.001] ro=0.0255427 weight=1
 - put full command line as a comment - useful for reproduction

multiple input files -- only when options -dfile and -o are not given

instrumental broadening - U, V, W, Gaussian/Lorentzian/Pseudo-Voigt
	       or instrumental pattern from file (like DIFFaX or PowderCell)

use link-cell algorithm without PBC if r_cut < r_max/4

optimize (id -> pattern) phase (FFT)

--continue option for id calculation (in case the previous job was interrupted)