M3 custom - should we assume identity links if none provided?

[venpopov]

As I'm thinking about how to implement the link functionality into the other models (#189), I stumbled upon how it currently works for the M3.

Currently we have the following check in check_model.m3_custom:

  stopif(
    length(model$links) < length(model$parameters) - 1,
    "Please provide link functions for all model parameters to ensure proper identification of your model"
  )

I left it that way during the refactoring of the PR, but at the time I wondered whether we should be less strict here. That is, instead of providing an error we assume that parameters have identity links (and perhaps turn this into a message?).