matrix traversal almost certainly incorrect

[ofloveandhate]

we need to ensure that Eigen::Mat are being traversed according to storage order. the default is col-major, but i think our uses of it are row-major... so yeah.

[ofloveandhate]

i fixed it in at least one place.

this doesn't make a huge difference right now, as other things completely overwhelm this in terms of time spent. but still nice to pay attention.