Allow iterate() to take arbitrary parameters

[atyre2]

Instead of passing parameters through a dataframe, ... would allow iterate() to take the parameters directly. This would enable e.g.

r <- rnorm(100, mean = sumry_griz$rbar, sd = sumry_griz$r_se)
# map_df() is a new function
bigresults <- map_df(r, ~iterate(N0=33, t = 1976:2007, b = ., d = 0, popfun=exppop), .id = "rep")

[atyre2]

This might be done by having methods, so the default method would take arbitrary parameters as above, but the data.frame / tibble method would use a parms object like presently.

[atyre2]

This is the same thing as #7 - would solve that too.