`psy6` branch: `iszero` on substring

[luke-kiernan]

I'm getting "MethodError: no method matching zero(::SubString{String})" when doing a roundtrip in PowerFlows.jl here. Probably need to add "if not numerical then Parse" logic.

[luke-kiernan]

Might be a false alarm. I tried adding that logic, and the error just changed to "cannot parse "" as Float64," which makes me think PF's PSS(E) exporter might be the problem.