[Python SDK] Add a Collection method to efficiently concatenate files
Concatenating N files together to create a new one can be accomplished with manifest text manipulation, without moving data.
- Add a method to the Arvados SDK that appends the segments of one ArvadosFile to another (a generalized version could replace a region of one ArvadosFile with a region from another one.)
- Provide example Python code (create a new ArvadosFile that is the combinations of two or more other files)
- Provide example CWL (requires passing in explicit keep references, talking to API server to create new output collection, and producing cwl.output.json with references to the new collection).