mc: Markov chain definition
In drlevy/ECS256-HW1: Markov Chain

Construct and initialize either a continuous or discrete markov chain. Input will be assumed discrete unless qidef is defined. Input will be assumed finite unless pijdef is a function.

1	mc(pijdef = NULL, qidef = NULL)

`pijdef`	is either a square finite transition matrix or a function(i, j) defining a (potentially) infinite transition matrix.
`qidef`	is a vector containing the holding-time of each state in a continuous markov chain or a function(i, j) defining a (potentially) infinite holding-time vector.

pijdef = matrix(rep(0,9), nrow=3)
pijdef[1,1] <- 0.5
pijdef[1,2] <- 0.5
pijdef[2,3] <- 0.5
pijdef[2,1] <- 0.5
pijdef[3,1] <- 1
markov_chain = mc(pijdef)