RuntimeWarnings thrown in centrality.py

[clbarnes]

Specifically, provoked by the unit tests in modularity_derived_metrics etc. by the functions participation_coef_sign.pcoef and diversity_coef_sign.

[aestrivex]

Cause of error warning is matrix-wide divide by zero since these metrics are computed separately for positive and negative component. Errors out if the matrix has no negative component but does the math correctly anyway (i.e., returns 0).

Error warnings suppressed for these functions.