## 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($b_y = \sum_xe_{xy}$)]].