ENH: Add sos conversion functions and tests
FIX: Calculate SOS ICs, better pairing
DOC: Tweaks to docstrings

Variables in `single backticks`
Code in ``double backticks``
Outputs are never array_like, they're either ndarray or list or etc.
some accept array_like inputs but were listed as ndarray
other small changes
FIX: Minor SOS algorithm fixes
BUG: signal: Fixed the scaling of the elements in sosfilt_zi.
FIX: Fix SOS algorithm and tests