SPZ conversions

[lucaferranti]

builds on top of #3015

implements conversion from Zonotope and SSPZ to SPZ and conversion from SPZ to SSPZ

[lucaferranti]

wtf? didn't I add the missing docstrings?

[schillic]

You need to add the where {N} precisely as in the method. (This is annoying because here it is redundant.)

[lucaferranti]

why the heck does the doc check keep failing?

│     Base.convert :: Tuple{Type{SimpleSparsePolynomialZonotope}, SparsePolynomialZonotope}
│     Base.convert :: Tuple{Type{SimpleSparsePolynomialZonotope}, AbstractZonotope}

[lucaferranti]

ah wait maybe I know

[schillic]

Yes, that should do it. I was confused that some method was old.