Only include easy conversion members on subclasses of Quantity that opt-in.
change Distance to be compatible with opt-in special members scheme
Only include easy conversion members on subclasses of Quantity that opt-in.
Merge pull request #14 from eteq/unit/special-members-on-quantity-subclasses-fix-distance

Unit/special members on quantity subclasses fix distance
Merge pull request #1326 from mdboom/unit/special-members-on-quantity-subclasses

Include unit conversion members only on subclasses of `Quantity`