Arvados

Concatenating N files together to create a new one can be accomplished with manifest text manipulation, without moving data.

1. 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.)
2. Provide example Python code (create a new ArvadosFile that is the combinations of two or more other files)
3. 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).