Improve documentation on ActiGraph csv files

[vincentvanhees]

In the vignettes we refer to Actigraph csv without providing much detail as if it is supposed to be obvious what we mean:
https://cran.r-project.org/web/packages/GGIR/vignettes/readmyacccsv.html
https://cran.r-project.org/web/packages/GGIR/vignettes/GGIR.html

To do:

• Add clarification what format GGIR expects by referring to the test file GGIR can create as example.
• Clarify distinction between actigraph count csv files and actigraph raw csv files, and point reader to documentation for processing count csv files.

[dsjoho]

This would be clarifying! Also for when one has a non actigraph file in order to determine what those files look like.

I'd also like to propose a clarification regarding https://cran.r-project.org/web/packages/GGIR/vignettes/readmyacccsv.html
As a new user, it is unclear to me if I can load an entire directory with the proposed read.myacc.csv solution, or if I'm then forced to only read single files. (This is probably obvious after a while, but initial reading is unclear)