Speedup create_test_acc_csv (as only used for unit tests)

[vincentvanhees]

Fixes #963 by speeding up the create_test_acc_csv.R function. The writing of the file was by far the slowest component and I have now replaced this by data.table::fwrite.

No need to mention this in changelog as it does not affect GGIR behaviour for the user.

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).
• Clean code has been attempted, e.g. intuitive object names and no code redundancy.
• Documentation updated:
  • Function documentation
  • Chapter vignettes for GitHub IO
  • Vignettes for CRAN
• Corresponding issue tagged in PR message. If no issue exist, please create an issue and tag it.
• 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.

[vincentvanhees]

merging now without peer-review, because only affects unit-testing and unit tests are all passing.