Provenance tracking

[joezuntz]

This adds provenance tracking to ceci stages and outputs.

• A Provenance class collects provenance information and comment information
• All stages generate their own provenance object, which stores:
  • Config options
  • The command line
  • The user and machine that ran the stage
  • Git status and diff information
  • Versions of all the loaded python modules
  • ID numbers for all input files
• The provenance is passed to all output objects, which can try to save them however they wish
  • Data handle classes should now inherit from a base class, which just handles this provenance
  • They can use the methods on the Provenance class to do so, mainly Provenance.write which knows about hdf, yml, pickle, and fits files for now.
  • They might instead want to use an external library to save things instead.
  • Or not save them at all if the file it too simple to be worth doing this for.

I'd like to think a little more about this before moving this from draft status.