Implement Dolph-Chebyshev / taylor window function

[Dantali0n]

In the Dolph-Chebyshev / taylor windowing function sidelobes can be supressed to -20 * a.

The math is a bit more involved then the window functions I added as pull request. It also requires changes to the function parameters as it would be nice to have a as a user definable parameter.

It would also be a good idea to add cosh as an private internal function since Dolph-Chebyshev relies on the cosh function.

I would like to hear what you think about this.

[kosme]

Changing the Windowing function parameters is a no no, it would break compatibility with existing code. But we could create a new function that accepts the new parameter and returns the data windowed according to Dolph-Chebyshev / taylor windowing.