Implemented is_multiplicative property in Definition and Converter. Removed delta_degK

- The OffsetConverter is multiplicative if the offset is 0.
- delta_degK is not needed because degK is multiplicative.
Refactor Quantity mul, div, and pow operators

Create methods_imul_div and _mul_div to centralize the multiplication
and division operator methods. Also make multiplication, division, and
power methods return NotImplemented when appropriate.
Improved support for non-multiplicative units.

Quantities with non-multiplicative units are converted to base units
before multiplication and division.

See Issue #86