Cycle Checks and Unreachable Nodes

[JackGeraghty]

There is no checking for cycles in the current version of the pipeline. The pipeline operates on the hope that the user doesn't try and create cycles in the DAG. It is possible to check for cycles so this check should be introduced.

It is possible to create Nodes that are not connected to any other node. This is typically a mistake by the user and can lead to time debugging. This could be avoided by performing a check when creating the graph and indicating to the user that a node is not connected.