Add Code to Preserve Metadata During Transformation

[jsmonson]

This PR updates qonnx with metadata preservation during transformation. This operates on the following rules:

1. If a new node is created alongside an existing node the information from the existing node is copied to the new node.
2. If a new node (or nodes) is (are) created to replace an existing node the metadata from the existing node is copied to the new node(s).
3. If a new node is created to replace multiple existing nodes (i.e. node fusion) author should select the most relevant node to source the metadata from. In this PR, the author made his best judgement on which metadata to preserve. Since there is no standard approach for fusing metadata from multiple nodes, this approach leaves it to transformation authors to determine which metadata to preserve.