Add central function to aid checking milestone folders

[vincentvanhees]

Fixes #1144

Function checks:

• Do milestone folder(s) exists as needed for current or preceding GGIR part(s). If not, it creates them.
• Are folders empty, if yes then give error telling the user what part(s) need to be run first.

This is useful when running part X when part X - 1 has not been run yet, which always produced cryptic error messages.

We already had some of this functionality but ad-hoc in some of the parts. In this PR I am moving all this to a central function to ensure we systematically do this for parts 1 to 5. I am skipping this for part 6, because part 6 is a bit different as it depends on threshold combination specific input folders.

Checklist before merging:

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

If NEW GGIR parameter(s) were added then these NEW parameter(s) are:

• [ ] documented in man/GGIR.Rd
• [ ] included with a default in R/load_params.R
• [ ] included with value class check in R/check_params.R
• [ ] included in table of vignettes/GGIRParameters.Rmd with references to the GGIR parts the parameter is used in.
• [ ] mentioned in NEWS.Rd as NEW parameter

If GGIR parameter(s) were deprecated these parameter(s) are:

• [ ] documented as deprecated in man/GGIR.Rd
• [ ] removed from R/load_params.R
• [ ] removed from R/check_params.R
• [ ] removed from table in vignettes/GGIRParameters.Rmd
• [ ] mentioned as deprecated parameter in NEWS.Rd
• [ ] added to the list in R/extract_params.R with deprecated parameters such that these do not produce warnings when found in old config.csv files.

[jhmigueles]

Changes look good to me and they work as expected in my tests, thanks!