Make dedicated error checking funtions

[Neves-P]

Quoting @richelbilderbeek #90:

) {
  testit::assert(
    "length(pars) is not five and/or shift_times is not null, set
    five parameters with no shift_times or ten parameters with
    non-null shift_times",
    length(pars) == 5 || (length(pars) == 10 && !is.na(prop_type2_pool))
  )
  testit::assert(
    "2 type islands cannot have species on the island initially",
    is.na(prop_type2_pool) || !is.na(prop_type2_pool) && nonoceanic_pars[1] == 0
  )
  testit::assert(
    "prop_type2_pool should either be NA for no type 2 species or value between
    0 and 1",
    is.na(prop_type2_pool) || (prop_type2_pool >= 0 && prop_type2_pool <= 1)
  )

I enjoy this error checking!

But first, this error checking is already quite complex. Use a function
for each check instead: this will be easier to debug, easier to read,
easier to write.

Second, it is getting clear that the function
is actually doing two things: check for 5 or 10 parameters. I suggest that
this function quickly redirects to functions specialized on 5 or 10 parameters.
Of course, there should be little code duplication, so things done for both
5 and 10 parameters should be put into little functions as well.