MNT : minor pep8 clean up
ENH : added classes for calibration data

 - HKL : subclass of namedtuple for dealing with hkl values
   - it _is_ a tuple
   - ensures values are always integers
   - provides convenience method length to take L2 norm
 - CalibrationAngles : class to hold 2theta calibration data
   - knows about:
      - lattice parameter
      - standard name
      - the two theta values
      - the hkl for those angles
      - the wave length used for measurement
   - protects data via read-only properties to reduce the chances of
     accidental mutation
   - provides convenience method `convert_2theta` which will return
     the angles at a different wave length

 - convert_two_theta function
   - math to calculate 2theta angles at other wavelengths.