Exogen variables as separate primary nodes
Created by: J3FALL
Instead of passing the exogen variables to chain with input data, it can be passed as a separate input for primary nodes.
Now: 1 InputData (with exogen) -> Chain Next: 2 InputData (source + exogen) -> Chain -> PrimaryNode1(source) + PrimaryNode2(exogen)
This can be done after #237 will be finished.