Add optimal fc plot

[dkobylianskii]

Summary

This pull request introduces the following changes.

• Add option to plot optimal fc value in HL fc_scan plot

This relates to the following issue

• 158

Conformity

• [x] Changelog entry
• [x] Documentation

[samvanstroud]

Thanks @dkobylianskii ! this looks great to me. Could you add an expected plot for the fraction scan with optimal_fc enabled? then this implementation will be also covered by the test.