Single state decision/chance nodes

[jdelab1]

Currently, the package doesn't allow the creation of chances of decision nodes with a single state

ERROR: DomainError with Each chance and decision node should have more than one state.

This can be useful in some cases, e.g., having a degenerate chance node to simulate different scenarios in parallel. I think it can be useful to just show a warning instead of stopping the code when this situation occurs.

[esahon]

Limitation removed in orderredict pull request

[toubinaattori]

v 2.0.0 contains a fix that enables single state nodes