`signal._filter_design`: improve `lp2{lp,hp,bp,bs}[_zpk]` and `bilinear_zpk`

[jorenham]

There are barely any github code search results for these, so this is low priority

[Aniketsy]

hi @jorenham as its low priority, please let me know, if i can help with this . Thanks!

[jorenham]

That would be awesome, thanks @Aniketsy.

For reference you could look at the overload patterns of similar functions like lp2lp (src) and tf2zpk (src).