Convert a plane angle from degree to radian.
First argument DEG is the number of arc degrees.
Optional second argument MIN is the number of arc minutes.
Optional third argument S is the number of arc seconds.
Optional fourth argument SIGN specifies the sign.
Value is the corresponding angle given in radian.
If two or more arguments are supplied, then MIN and S are sexagesimal
subdivisions of an arc degree. In this case, DEG, MIN, and S have to
be non-negative numbers and the sign of SIGN, after conversion into a
floating-point number, determines the sign of the angle.