Small fixes related to loading diaries and storing part 2 output

[vincentvanhees]

• Correct identification of number of files for part 2 generation
• Further improvements to handling dates in sleeplog as a follow-up to work on Speeding up and making date format recognition more robust in function g.loadlog #1243
• Both activity diary and sleeplog are now always reloaded if diary .RData file is older than diary itself.
• Make sure event diary is save to csv with intended sep and dec rather than default.

Note: I do not update documentation because:

• The logic of re-loading activity diaries is only for internal workings of GGIR and not of direct relevance to user.
• The other points concern bugs fixes and not functional changes.

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).
• Clean code has been attempted, e.g. intuitive object names and no code redundancy.
• Documentation updated:
  • Function documentation
  • Chapter vignettes for GitHub IO
  • Vignettes for CRAN
• Corresponding issue tagged in PR message. If no issue exist, please create an issue and tag it.
• 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 file, if you think you made a significant contribution.
• GGIR parameters were added/removed. If yes, please also complete checklist below.