rsetienne/DAISIE: v4.0.0

DAISIE 4.0.0 N.B.: MacOS users may experience issues when installing DAISIE, especially when on MacOS Big Sur. If that is you case, please see here for detailed installation instructions. Fix bug when calculating conditional probabilities, which are now correctly calculated from island age to the present. Fix bug when calculating probabilities upon migration, which assumed no recolonisation was possible in the CS model. Handling recolonisation in the same manner is not possible for the IW model, so an approximation is now made. The influence of the bug and of the approximation in the IW model is expected to be minimal, particularly in cases where the colonisation rate is low. Such cases of low colonisation are the norm. Add num_cycles argument to ML functions, allowing to specify how many cycles the optimizer should take. Defaults to 1. Minor vignette corrections. Improve README.md documentation