Prototype for provenance tracking mechanism

[huard]

This builds on @dcherian work to track history and cell_method and adds basic provenance tracking.
#228 (comment)

It's inspired by what metaclipR is doing.

Things to do:

• xarray: grab more context information for function calls (dimension, arguments)
• Add nodes for call arguments ds:Argument
• Inference mechanism to detect relevant terms in an ontology. For instance, CMIP5 is defined, so when we find a project or project_id called CMIP5, we should try to refer to it instead of creating a new ds:Project entry as is done here.
• Make sure the logic works when multiple functions are chained.
• API to save (serialize) the graph in an external file when dataset is written to disk.

I'm hoping someone can take over for a while because I won't be able to invest a lot of time on this in the near future.