added implementation of Van Hateren 2007 model
Merge branch 'master' of https://github.com/bps10/base

* 'master' of https://github.com/bps10/base:
  added gaussian blur
  added wavelen to cones, wav2RGB heuristic
  changed strehl ratio
  minor input change in psf
  added cones, change imports
  moved mtf, psf and diff methods here