generate_report fails when less than three optimizers due to Friedman testing

[andreas122001]

The script carps.analysis.generate_report fails on critical difference plotting if less than three optimizers are given since Friedman test requires multiple samples.
E.g. error:

ValueError: At least 3 sets of samples must be given for Friedman test, got 2.

Todo: change the generate_report function to only do reports depending on Friedman tests if there are more than two optimizers being compared.

Could be something like wrapping it in an if-statement

n_optimizers = len(df['optimizer_id'].value_counts())
if n_optimizers >= 3:
    # critical difference
    # ranks over time
    # etc.