facilitate double backward slashes + add verbose + couple of other minor updates

[vincentvanhees]

This PR addresses a couple of minor issues in preparation for the 2.9-0 release. Although, for now I have kept the version number at 2.8-7. In that way we can make 2.8-7 release this week and then leave a bit of time to test more extensively before the 2.9-0 release later this month.

Fixes #721 => Filepathes being stored were incorrect in RData file, now fixed, even though GGIR itself does not use them.
Fixes #570 => We now facilitate also double backward slash for file and folder paths
Fixes #704 => GGIR version is now better recognised for developers who have not installed GGIR.
Fixes #652 => Argument verbose added to ease turning off console messages.

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.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@d0aeaf9). Click here to learn what that means.
The diff coverage is 73.10%.

❗ Current head 264f2ef differs from pull request most recent head 7cbdfb5. Consider uploading reports for the commit 7cbdfb5 to get more accurate results

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@@            Coverage Diff            @@
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  Coverage          ?   74.25%           
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  Files             ?       88           
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Impacted Files	Coverage Δ
R/g.report.part2.R	65.40% <25.00%> (ø)
R/g.part3.R	68.75% <40.00%> (ø)
R/g.part4.R	75.18% <40.00%> (ø)
R/g.plot5.R	82.15% <50.00%> (ø)
R/check_params.R	74.87% <55.55%> (ø)
R/g.part2.R	60.75% <57.14%> (ø)
R/g.part5.R	76.02% <60.00%> (ø)
R/g.part1.R	67.60% <70.96%> (ø)
R/g.analyse.perfile.R	93.39% <75.00%> (ø)
R/g.report.part5.R	84.11% <75.00%> (ø)
... and 3 more

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[jhmigueles]

Hi @vincentvanhees, just did a minor commit to handle the verbose option in the report part 2 and report part 4 (messages were printed even if verbose = FALSE). The rest of the PR looks good to me!