Improve grouping of zeros with poles for SOSFilters

SOSFilter conversion now checks that complex poles and zeros form
complex conjugate pairs, and I've tweaked the algorithm for pairing so
that it matches MATLAB and added tests.
Fix a type instability issue in SOSFilter conversion and add asserts
Don't cancel equal zeros/poles on ZPKFilter construction
Merge pull request #67 from JuliaDSP/sjk/sos_pairing

Improve grouping of zeros with poles for SOSFilters