Extract GENEActiv starttime from first page header.

[vincentvanhees]

This PR:

• Fixes GENEActiv: when starttime is "2010-09-16 09:10:00:000" use first page time instead. #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 GENEActiv: when starttime is "2010-09-16 09:10:00:000" use first page time instead. #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]

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.

❗ 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 ⚠️

Additional details and impacted files

@@            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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