Suggestion: Warn user when ddep = 0 and K != Inf

[Neves-P]

In the DAISIE_ML() functions, the user can specify the type of diversity dependence (DD) to be considered by setting the ddep argument to different integers.

ddep = 0 by default, results in no diversity dependence being considered in the MLE. However, a user can still specify a "valid" value of K to the estimation routine, which implies that diversity dependence is being considered. This is compounded by the fact that in the DAISIE_sim() functions, DD is specified by setting K in pars to be != Inf. It is easy for a user to mean DD in the routine, but leave ddep = 0 by mistake as it is the default (see Neves-P/DAISIErobustness#43). I suggest we add a warning() when ddep = 0 and K != Inf in initparsopt to mitigate this issue.