function documentation examples

[juniperlsimonis]

from the LDATS code review, which I think we should follow here:

Please add small executable examples in your Rd-files to illustrate the use of the exported function.

\dontrun{} should be only used if the example really cannot be executed (e.g. because of missing additional software, missing API keys, ...) by the user. That's why wrapping examples in \dontrun{} adds the comment ("# Not run:") as a warning for the user.
Please unwrap the examples if they are executable in < 5 sec, or create additionally small toy examples to allow automatic testing, (or replace \dontrun{} with \donttest{}).

When creating the examples please keep in mind that the structure
would be desirable:
\examples{
examples for users and checks:
executable in < 5 sec
\dontshow{
examples for checks:
executable in < 5 sec together with the examples above
not shown to users
}
\donttest{
further examples for users; not used for checks
(f.i. data loading examples )
}
}

[juniperlsimonis]

addressed with #129