Check all parameters

[Neves-P]

Quoting @richelbilderbeek #90:

Not all input parameters are checked. For example, for the diversity
dependence model: there is no check if a
valid model is used (e.g. testthat::expect_true(divdepmodel %in% c("IW", "CW", "GW"))).
If the XXX (yes, XXX indeed) diversity dependence model is used, there
is no helpful error message. Add some functions
like assert_div_dep_model/check_div_dep_model to check a
divdepmodel for validity or use get_all_div_dep_models to get all valid
models.