Add test for cosine decay

[florence-bockting]

Description

• add additional tests for testing
  • function value and derivative at harmonisation time and convergence time
  • harmonisation time > convergence time
• adjust CosineDecaySplineHelper to support the case harmonisation-time > convergence-time
• add how-to-guide for usage of cosine weight-decay

Checklist

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Changelog item added to changelog/