Replacement for @raoulb pr#914

See #914

ToDo:
- [x] tests
- [x] final rebase
On Fri, Jul 18, 2014 at 05:50:32PM -0700, raoulb wrote:
>  > +class asin(InverseTrigonometricFunction):
>  > +    """
>  > +    The inverse sine function.
>  > +
>  > +    Returns the arc sine of x (measured in radians).
> 
>    This is not really true:
>    sin(x [radians]) --> number [no unit]
>    or maybe it is:
>    [1]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.

Perhaps, the part part in parentheses is about of the return value
of asin(x), not about x.  And this is - an angle.

Do you suggest a better wording?