bpmodels v0.2.1

Minor functionality change

• chain_sim() now throws a warning, instead of an error, when tree is set
  to FALSE with serial also specified. We assume that providing a serial interval
  means you want the tree of transmissions to be simulated, so chain_sim()
  internally sets tree = TRUE and throws a warning explaining what happened.
  This behaviour should not break any simulations with previous versions
  with bpmodels, but if it does, please submit an issue.
  To remove the warning, the user should explicitly set tree = TRUE when
  they specify serial.