Simplify --resume interface

[rickyegeland]

The help of sphinx.py currently reads:

  --resume              Specify resume to add forecasts to an existing
                        dataframe and recalculate metrics.
  --DataFrame DATAFRAME
                        Filename of existing Pandas DataFrame containing the
                        results of a previous SPHINX run in pkl format. Must
                        specify if choose resume.

Simplify this by requiring the DATAFRAME argument is passed to --resume, e.g.

python bin/sphinx.py --resume SPHINX_dataframe.pkl

This presumes there is no use-case for running with --DataFrame and without --resume.

[ktindiana]

Resolved with commit 942b3f1