Add optional cosinor analysis to part 6

[vincentvanhees]

In a project I am working on there is a desire to apply the cosinor analyses to only the time series used in the analyses for part 6

Draft of action items:

• Move g.analyse.avday code section related to cosinor analyses to new function to ease re-using the whole procedure in part 6
• Add function to part 6 and update part 6 to make sure it works.

TO DO:

• Add cosinor estimates to part 6 reports
• Expand documentation
• Test
• Add option to skip cosinor analysis in either part 2 or part 6.