Revision transition probability calculations and clean up function g.fragmentation

[vincentvanhees]

In this PR:

• g.fragmentation code cleaned up
• TP formulas revised following feedback Ian Danilevicz
• Code now also produces sleep fragmentation and PA fragmentation for the SPT window
• All fragmentation variables are not automatically rounded to 3 but to 6 decimal places as we expect some fragmentation
• metrics to reveal meaningful differences at 4 or 5 decimal places

Relates to #817

@jhmigueles - When you have time in the upcoming weeks, would you mind testing that this branch still works correctly for the new part5 segment analysis as discussed here?

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.

[jhmigueles]

I can confirm that this works in combination with segment analysis in part 5. NA values in fragmentation metrics are now handled and values are virtually the same. I noticed that for some days/segments, in the new version we get one less fragment than in the current version in master, which I think is in line with this revision of the function.