Fix sorting of multiple keys and misc

[Vindaar]

Mainly fixes a bug messing up sorting by multiple keys. The last row was not sorted and remained the unsorted last row.

Note to self: should spend more time writing tests...

Full changelog:

* v0.3.20
- fix a bug that causes the =arrange= proc to mess up sorting if more
  than one key is used. The last row of the DF was not sorted before
  and thus remained in unsorted order at the end of the sorted result.
- =geom_errorbar= now takes an =errorBarKind= field to set the kind of
  errorbar to draw
  - =ebLinesT=, =ebLines= are the only two options atm
- add =scale_fill_continuous= to force the fill scale to be
  interpreted as continuous values
- =ggsave= will now create the directories contained in the path given
  to it instead of silently not creating a plot if the path to the
  given file does not exist yet.
- attempt to get rid of duplicate legends in case of certain ggplot calls

[Vindaar]

Did I seriously manage to bake in bugs into the tests or did I also break something at the same time?

Will investigate in the coming days.

facepalm: I guess I did. Who decided I get to code this?
edit2: oh no, haha. I didn't after all. I just confused the buggy number with the tests number. So I only broke something after all.
edit3: fixed it. Nothing too bad, just me misunderstanding my own code... :)