Summary plots are broken

[ns-rse]

Whilst working through parametrising the changes proposed in #937 to tests/test_utils.py::test_create_empty_dataframe() I noticed that the renaming of columns in the dataframes has also broken the summary plots that are, by default automatically generated as the log includes the following messages...

EDIT : Only two variables are not present under their old names.

INFO     topostats:plotting.py:409 [plotting] Statistic is not in dataframe : contour_length
INFO     topostats:plotting.py:409 [plotting] Statistic is not in dataframe : end_to_end_distance

This is mostly down to renaming of variables in the grainstats_df or results_df data frame.

This needs fixing and ideally we should put tests in place to ensure that these plots are generated correctly so that future changes are captured and do not rely on serendipity to be caught.