Investigate issues with capturing vertices' exposure at time of adoption with `node_threshold()`

[henriquesposito]

Although our node_threshold() function is more flexible (i.e. works on various types of networks), results are not consistent with those of netdiffuseR. We should investigate and fix or explain why this happens.

For example:

library(netdiffuseR) c(netdiffuseR::threshold(medInnovationsDiffNet))

vs.

library(manynet) unname(c(node_thresholds(as_diff_model(medInnovationsDiffNet))))