respect function dfilter defaults

Another (actual) fix to #354 (closed)

First of all sorry! I committed the changed image files and while I was suspicious, I never actually checked the figures. Doing so would have revealed the problem immediately.

I dug into #354 (closed) and the problem is (again) caused by shadowed function default values. plot depends on dfiler=FILTER_NONE in order to show non-empty plots. When a plot call is routed through core.modules.tools.plot (as is the case with the change suggested in !494 (closed)), we pick up the dfilter default defined there. If we don't (as is the case in the current develop) we explicitly set dfilter to the translation scheme's DFILTER_DEFAULT in saqc.core._wrap.inner. And this DFILTER_DEFAULT happens to be set to saqc.constants.FILTER_ALL, leaving as with all empty plots.

So the necessary fix to the problem is to not overwrite dfilter defaults, by checking first, if they do exist (inspect to the rescue...)

closes #354 (closed)