v0.18.8

MLJBase v0.18.8

Diff since v0.18.7

• Fix bug with Resampler (#559)
• Add train_test_rows field to performance evaluation objects (#559)
• Improve error message for attempts to call report/fitted_params on an untrained machine (#558)

Merged pull requests:

• Add more informative error for report/fitted_param on untrained mach (#558) (@ablaom)
• Ensure Resampler does not generate new train/test pairs unless resampling or repeats changes (#559) (@ablaom)