Add support for assortativity based on scalar properties
Add support for assortativity coefficient based on scalar properties [[Equation(x
)]] and [[Equation(y
)]], defined by
[[Equation([ r = \frac{\sum_{x,y} xy (e_{xy} -a_x b_y)}{\sigma_a\sigma_b} ])]], where [[Equation(a_x = \sum_ye_{xy}
)]] and [[Equation(b_y = \sum_xe_{xy}
)]].