Store input file data labels in pybamm.Experiment format

[sjrowlinson]

Process the data labels from the input files, and convert them into the statement syntax used to construct pybamm.Experiment objects.

• [ ] Biologic
• [ ] Ivium
• [ ] Maccor

[mjaquiery]

This seems like a good addition; I know the AmpLabs people were interested in PyBAMM as a central format. I'm not quite clear on what we want witht this issue or where it fits in, though.

• Should we be storing file metadata in pybamm.Experiment format (or a serialized instance of it) in our own back end?
• Are we exposing a mechanism for users to export our data in pybamm.Experiment format?
• Is this something that should be done to all files at parse time, or is it something we want to ensure we can do for files we've stored?
• How might we implement this in the web frontend?

[mjaquiery]

Need to pick this back up on the feature/pybamm_labels branch when ready.