Add helper functions that return risk / vaccination group indices

[lwhittles]

We already have get_age_bins()

mpoxseir/R/parameters.R

Lines 118 to 125 in 0357e13

	get_age_bins <- function() {
	end <- c(seq(4, 75, by = 5), 100)
	start <- c(0, end[-length(end)] + 1L)
	label <- paste(start, end, sep = "-")
	label[length(label)] <- paste0(max(start), "+")
	data.frame(label = label, start = start, end = end)
	}

Which can be used to get names / indices for those, but nice to expand to include SW and PBS as we use this everywhere

Ditto vaccination, so we can update the code easily if we re-order the strata for any reason