lingering comments

[loftusa]

on eric's end (I didn't look at my own comments - maybe eric finds more after going through #92)

• [x] ch5 models with covariates line 144: change to latter notation here. go back to multi-network section and make them look the same as this
• [x] ch5 multi-network-models line 62 ericwb95: overloaded notation! >:( >:( >:( capital letter
• [x] ch6 multigraph.tex line 103 "this whole box seems redundant" discussion
• [x] ch7 sparsity.tex line 312 look at bpedigo's comment about csr_array
• [x] ch7 sparsity.tex line 398 figure discussion
• [x] next/diffusion.tex line 347 repurpose figure that appears later on