Demo how to use `matched.distances()` in vignettes

[benthestatistician]

There should be a passage of a vignette explaining how to use matched.distances() to calculate the sum or mean of matched distances. One use of this is to determine matched distances along the distances used for matching; another is to determine matched distances along a different distance.

[benthestatistician]

Notes:

• It calls for e.g. sum( unlist( matched.distances(mymatch, mydist) ) ), which has at least 2 nonintuitive aspects (the need to pass a distance; unlist()).
• One natural place for this passage is in or after the "Distances" subsection of fullmatch-vignette.Rmd.