node_value_to_index: An auxiliary function to mml_nb_adaptive()
In kelvinyangli/mbmml: Learn Markov blankets using Minimum Message Length

This function converts the variable values to an index in the set of all combinations. For example, the values c("A","A","A") is converted into 1, c("B","A","A") is converted to 2.

1	node_value_to_index(arities, values)

`arities`	A vector of arities of the Xs and Y. It is in the order of c(xIndices, yIndex).
`value`	A vector of variable values in the order of c(xIndices, yIndex). The values are in numeric format.