'get_interactions' is modified to process vectorized input

Note that when both 'taxon' and 'interaction.type' are vectors, output represents 1 to 1 correspondence of values. i.e., with taxon = c("a","b"); interaction.type = c("c","d"), function returns data for (a,c) and (b,d) pairs, which may be confusing. to make it return (a,c),(a,d),(b,c),(b,d) instead following lines should be added:
taxon <- rep(taxon, each = length(interaction.type))