rename.

--HG--
rename : arsenal/unit_conversion/__init__.py => arsenal/units/__init__.py
rename : arsenal/unit_conversion/data.py => arsenal/units/data.py
rename : arsenal/unit_conversion/main.py => arsenal/units/main.py
rename : arsenal/unit_conversion/option.py => arsenal/units/option.py
rename : arsenal/unit_conversion/unitatom.py => arsenal/units/unitatom.py
rename : arsenal/unit_conversion/unitdata.py => arsenal/units/unitdata.py
rename : arsenal/unit_conversion/unitgroup.py => arsenal/units/unitgroup.py