Clean up g.part5()

[vincentvanhees]

Fixes #871
In this PR I am moving large sections of the g.part5 function code to two new functions:

• One new function for all analyses per segment
• One new function to initialise data.frame ts

Note:

• This does not include any work on circadian rhythm yet, it is only preparation work for rest of the year.
• This does not include any other functional changes. I only copied code, checked that the tests still pass, and documented the new functions.

Checklist before merging:

• Existing tests still work (check by running the test suite, e.g. from RStudio).
• Added tests (if you added functionality) or fixed existing test (if you fixed a bug).
• Updated or expanded the documentation.
• Updated release notes in inst/NEWS.Rd with a user-readable summary. Please, include references to relevant issues or PR discussions.
• Added your name to the contributors lists in the DESCRIPTION and CITATION.cff files.