hybrid_fun: Hybrid landmark function
In niklasmaltzahn/multistate: simulation and estimation of multistate models

Function extracting a subset of a multistate data frame based on a landmark time and state. However, only information in non markov transitions are as in the landmark sample. The rest of the transitions contains same information as in the original multistate data frame.

1	hybrid_fun(data, s, l, non_markov_trans)

`data`	Multistate data frame of type from sim_fun.
`s`	Landmark time.
`l`	Landmark state.
`non_markov_trans`	Transitions for which only landmark data should be kept.

##---- Should be DIRECTLY executable !! ----
##-- ==>  Define data, use random,
##--	or do  help(data=index)  for the standard data sets.

## The function is currently defined as
function (x)
{
  }