[SciPy-Dev] Signal: Add type III/IV filter calculation to firwin2

[Andrey Smirnov]

Hello everybody,

Since current version of firwin2 can only produce type I and type II
filters as a result, and circumstances where type III/IV are needed are
not unthinkable, I decided to implement said functionality. The results
can be seen at:
    https://github.com/scipy/scipy/pull/39

I would really appreciate your comments on the code itself and on
whether it is possible to include this functionality into scipy.

Thanks in advance,
Andrey Smirnov