add tablespoon, teaspoon and drop
add warning about second and min being redefined.
use non-greedy operators to simplify regexps
fix radians: add a unit nonethless.

This ensure type safety to avoid adding radians with some non-radian but
dimension-less constant.
Plus it keeps information of what the unit is, in case it is written to a file.
(because degrees are dimension-less too)
fix steradion too...
add warning about drop