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If DISPLAY environ not found, then default to the Agg matplotlib
backend. Useful for running on clusters etc.

Closes #68 by giving a simple interface to set the log-level (unless set
at the script level) and whether or not to use cached data.

During setup, this writes a file tupak/.version which contains the git
hash and change date. We can extend this in future to contain other
information. This is then printed when the logging is initialised.

If a label is given to setup_logger, the output is written to a file.

- Adds utils function to facilitate writing logging output to the
  terminal
- Adds this to the basic tutorial