Replacement for @raoulb pr#914

See #914

ToDo:
- [x] tests
- [x] final rebase
This is not really true:
`sin(x [radians])  --> number [no unit]`
or maybe it is:
http://en.wikipedia.org/wiki/Radian#Dimensional_analysis
I'd still say that the angle is in radians but the result of `sin` is not.

Here it is rather a mathematical statement on the unit of computation. So it might be ok.