Improved support for quantities with offset units

- Wrote unittest for results of all possible operations (all T-units with
  operators add, sub, mul, div ang their inplace-versions)
- Remove delta for absolute T units (dropped dimension [delta_temperature])
- Updated unit tests as required because of the delta_kelvin, delta_degR
  removal
- Implemented new behaviour in addition and subtraction 
- Restricted convert to unambiguous cases if offset units are involved. 
- Added more tests for converting especially for compound units.
- Implemented new behaviour in multiplication and division. 
- Made automatic conversion on mul/truediv to kelvin configurable by means 
  of a new ureg-attribute: "autoconvert_offset_to_baseunit" (default: False)
- Modified tests to test with ureg.autoconvert_offset_to_baseunit = True
  and ureg.autoconvert_offset_to_baseunit = False
- Test test_issue86c changed to expected failure.
- Fix 1./Q_(0., 'degC') ZeroDivivsion error (changed `__rtruediv__`)
- Added tests for multiplication / division by scalar.
- Added tests for higher order offset units.
- New methods/properties for quantity objects to help with offset units.
- Modified pow and ipow for compatibility with offset units & added tests.
- Added __rpow__ method for _Quantity and tests for pow/ipow/rpow. 
- Updated documentation

Solves issues #147, #155, #144, #88