specifying no transmission in `offspring_opts()`

[sbfnk]

At the moment specifying no transmission from those isolated (as in the Hellewell et al. paper) requires specifying isolated = function(n) return(0) which is perhaps not very intuitive - should we accept a NULL option that does this for the user?

[joshwlambert]

Yes, this looks sensible. As isolation is a hard coded intervention, for now I think it's better to make the user specify it as a zero-generating function. But once we address #137 then allowing offspring distributions for groups that cannot occur because that intervention is turned off to be specified as NULL is a simple solution.