Increment time correctly between pages GENEActiv

Fixes #61

This should not impact GGIR usage, because GGIR only uses first timestamp, but will impact other usage of GGIRread::readGENEActiv.

Checklist before merging:

[x] Existing tests still work (check by running the test suite, e.g. from RStudio).
[x] Added tests (if you added functionality) or fixed existing test (if you fixed a bug).
[ ] Updated or expanded the documentation.
[x] 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.

wadpac / GGIRread