building vignettes with devtools::built() failed as
    there is no ogs executable in the build path
    vignettes were knitted with knitr only

    unused examples removed
    examples that are described in vignettes removed

future, just getting used to workflow again)

added minor corrections to functions after noticing some things in the
benchmark files ("name" attribute of gml points
can also be something else than "origin" now, point tibbles without a
"name" column are now accepted, polylines can now have more than 2
points and are still written to xml correctly)

user_input.R, first one contains S3 methods and second one is a small
test that should be ready to run (if the r2ogs6 package is installed and loaded)