Transitions specification

[choisy]

To make user interface even smoother, I would not force the user to specify NULL values explicitly: whenever a distribution is not specified explicitly, the program has to understand that it's Markovian. I would thus replace

distributions <- list(
  S = NULL,
  I = exponential(rate = 0.5),
  R = NULL
)

by

distributions <- list(I = exponential(rate = 0.5))

Furthermore, whenever the elements of distributions are all of the same type, I would also allow the user to specify it with a vector instead of a list:

distributions <- c(I = exponential(rate = 0.5))

[thinhong]

I've fixed this