DOC: Added Ni known d spacing
DOC: added CeO2 (hkl) values (angles) values and wavelength
DOC: added Alumina(Al2O3) (hkl) values and (twotheata) values for XPD
DOC: modified nsls2/constant.py
Merge pull request #169 from sameera2004/add_constants

add_constants : Calibration Standards for Powder diffraction data