[master 20170130 E2] 'chemmatrixaux.py' (Rev. 4)
[master 20170130 E2] 'chemmatrixaux.py' (Rev. 4)

- Editted dx_pbc to subtract/add L instead of L/2 in case of images
- Changed d_dx_pbc to take derivative of correct index 'l' instead of
  'k'.

KhunWasut/chempython