MNT: Made xraylib optional
MNT: Cleaned up package list in setup.py
MNT: Replaced package with the correct one
MNT: Refactor of constants.py

- Created constants sub-module
- Created api.py file in constants submodule that provides a
  clean entry point into our constants objects/functions
- Created BasicElement class that holds basic element information
- Created XrfElement class that subclasses BasicElement and adds
  X-ray fluorescence information
- Moved Xray scattering calibration stuff into xrs.py module. Will
  probably end up moving that somewhere else

MNT: Removed neutron.py from constants/
MNT: Fixed basic element creation to include more constants

MNT: Fixed link direction based on caswell comment
MNT: BasicElement now working and passing all tests
MNT: All tests passing with XRF refactor

MNT: Cleaned up imports

MNT: Fixed tests based on removal of scattering factors
ENH: Added atomic constants data