Variable mTry at each node

[ArashBayatDev]

following issue #154 (Variable training parameter)

if the mtry parameter (for each tree or each batch, or the whole forest) is an array so that the first mtry applies to the root node. the second mtry applies to first-level nodes and so on.

if there are more levels in a tree than the length of mtry array then the last mtry is used for all the remaining levels.

[rocreguant]

Currently no need. #lowPriority