Use `iszero(?)` instead of `?==zero(...)` for warning when adding edge, for Symbolics Num compatibility

[kockahonza]

A partial fix/workaround for #49 with no downsides as far as I can tell. It is just a replacement of comparing to zero via == to using iszero. Though this isn't a comprehensive change to make all things work with Num types, it at least allows their creation (and they seem to mostly work in my usage).