Extract GENEActiv starttime from first page header.

[vincentvanhees]

This PR:

• Fixes #56. A client informed me that they corresponded with the manufacturer of GENEActiv and learnt that the start time reported in the file header becomes "2010-09-16 09:10:00:000" by default when recordings stops prematurely because of batteries running out (as described in #56). However, the timestamp in the first page header is always correct and I now use that as backup option if start time of file header matches this problematic date and time. Client has confirmed that this branch addresses the issue.
• Additionally, I am adding a PR template message which did not exist yet.
• Codecov yml edits to make it less strict about code coverage changes.

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (5a791ea) 90.01% compared to head (72db846) 89.82%. Report is 1 commits behind head on main.

:exclamation: Current head 72db846 differs from pull request most recent head 1daeb84. Consider uploading reports for the commit 1daeb84 to get more accurate results

Files	Patch %	Lines
R/readGENEActiv.R	33.33%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #57 +/- ## ========================================== - Coverage 90.01% 89.82% -0.19% ========================================== Files 9 9 Lines 901 904 +3 ========================================== + Hits 811 812 +1 - Misses 90 92 +2 ```

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