Speed up link_analysis_coarse

[EwoutH]

Speed up link_analysis_coarse, since it takes up the majority of runtime.

@toruseo you can assign me to this issue.

[toruseo]

ok, plz try

but it is a single for-loop. Is there much room for improvement?

[EwoutH]

Vectorization can help in many cases.

But now that we only run it once #130 instead of multiple times another approaches, this is less high priority.