Fix error while generating odin model

[richfitz]

This PR fixes the error seen by Debbie - it's not terribly obvious at first:

      nm <- deparse(e[[1L]])
      ...
      if (nm %in% c("length", "dim") && length(e) == 2 && is.symbol(e[[2]])) {

this produces a length 2 logical where deparse returns a length 2 vector. We assume it won't do that because normally we use it on sensible calls. However we also use this in the generation phase where we just want to peek into the rhs to see which index variables (i, j, k etc) we use and by that time we've rewritten binomial draws as

OdinStochasticCall(sample = "binomial", mean = S_leave[i, j, 
    k] * (S_rates[i, j, k, 1]/S_leave_rate[i, j, k]))(S_leave[i, 
    j, k], S_rates[i, j, k, 1]/S_leave_rate[i, j, k])

but here the "function name" (the thing called), is OdinStochasticCall(sample = "binomial", mean = S_leave[i, j, k] * (S_rates[i, j, k, 1]/S_leave_rate[i, j, k])) which flows onto two lines in deparse...