Refactor graph copy functions

[Affie]

Look at draft of copyGraph functions in attic/sandbox.jl
The idea is to have only one function to be called by functions such as buildSubgraphFromLabels

Will at least look at it for v0.6.0 and decide if it should be added.

[Affie]

related to #250
Will refactor and consolidate in the v0.6.x cycle

[dehann]

agree, buildSubgraphFromLabels should be using common DFG functions to minimize code maintenance. Might need a keyword or two for more use cases.