Fluorescence trace offset correction from configuration

[paulmueller]

Future versions of Shape-In and DCKit will make use of the configuration key ["fluorescence"]: baseline 1 offset (also for 2 and 3) to determine the actual trace offset correction (done online or in post-processing via DCKit). This offset should also be added to the trace visualization in Shape-Out 2.

Note for first contributors: This requires these configuration keys to be implemented in dclab.definitions. @maxschloegel is working on this in the context of DC-analysis/DCKit#11.