Use units.
Add units to some Element  properties. Add int, float multiplciation for units
so that the units are retained.
Add more doc and unit tests for units.
Add units to ionic radii.
Updated docs.
Add decorator to assign units to function output.
Add units to final energy of Vasprun and Oszicar.
Add div and truediv support in units.
Improve implementation of units.
More units.
Support for lists and tuples in unitized.
More doc.
Make it less easy to modify internal unit variables.
Finetune unit implementation.
REmove useless conversion factors.
Cleanup molecule matcher.
Merge remote-tracking branch 'mpmaster/master'

Conflicts:
	pymatgen/core/units.py
 (Almost) final version of ArrayWithUnit