Changes to current_flow_closeness_centrality

PEP-8 changes, disabled normalization (kept it as a keyword argument for legacy support), and removed duplicate code.

Normalization is a problem here. The docstring says that normalization results in a 1/(n-1) modification, but this isn't borne out by the code: it's actually multiplying by (n-1). 

Furthermore, it appears that normalization isn't necessary with this particular algorithm, as it inverts the core centrality calculation (resulting in correct behavior). This has the effect of producing results in the range [0,1] when weights are default in any case. What would be normalized?