Clarify minimum recording duration in vignette

[vincentvanhees]

Note urgent, but just leaving this here as a note for later this year.

From the training it became clear that it is not clear what the minimum recording duration is for GGIR to consider.

This happens in two steps:

• First there is a check that raw data file is at least 2MB in function g.part1(), where 2MB can be checked with argument minimumFileSize. Ideally this should never be changed, it is just there in case anyone believes that less data is also sufficient.
• Secondly, the code checks whether has at least two long windows (default 15 minutes) in g.readaccfile, which can be altered with the second value of argument windowsizes.

It would be good if this is clarified somewhere in the vignette.