Support for Frac, Sign and Fract

[ronnycsharp]

Hi Dmitry,

it would be nice to have these functions.
I've already implemented the fract-expression which
converts a double into its fractional parts and returns a Div-expression.
It's in the dev-branch of my fork.

Regards.
Ronny

[ronnycsharp]

perhaps it would also be nice to have a trunc function with two parameters, like this.
https://en.wikipedia.org/wiki/Truncation