Deprecate init_cumulative_fit

[sbfnk]

          we should depreciate this at some point. I don't think it helps and its just code to carry around

Originally posted by @seabbs in #537 (comment)

[sbfnk]

In the first instance this needs a deprecation warning similar to the one e.g. in get_dist:

EpiNow2/R/get.R

Line 182 in 49473e8

get_dist <- function(data, disease, source, max_value = 14, fixed = FALSE) {