plot_popkin OOM for larger kinship matrices

[mkelcb]

The plot_popkin function generates OOM when the number of individuals in the Kinship matrix is large. I was able to generate a plot with a RAM of 250GBs with an n=12000, but for larger matrices (eg 30-70K) this generated an OOM error.

Maybe using some form of chunking (like you do for the main kinship matrix calculation) you could get around this issue. (My workaround atm is that I simply to randomly downsample my Kinship matrix to 12K before plotting it.)