This function subsets a distance matrix based on specified sites
See also
[Based on Thomas Keggin's implementation for gen3sis](https://gitlab.ethz.ch/ele-public/gen3sis_wiki/-/blob/master/tools/keggin/distanceSubset.R)
Examples
if (FALSE) { # \dontrun{
# get and subset a distance matrix
distance_matrix <- readRDS(system.file("extdata/TestSpaces/geodynamic_spaces/raster/distances_full/distances_full_4.rds", package = "gen3sis2"))
distance_subset(distance_matrix, site_vector = c("7","15"))
} # }