updated WW5CFit to not use firstEvent() function of Marlin processor

[JennyListDESY]

BEGINRELEASENOTES

• since Marlin::Processor:firstEvent() is not usable with MarlinWrapper, the booking of histograms has been moved to init()
• this also required declaring their pointers as data members instead of the static declarations in the .cc

ENDRELEASENOTES

[tmadlener]

Hi @JennyListDESY, sorry this took a bit longer, but after key4hep/k4MarlinWrapper#92, isFirstEvent should work as expected in wrapped processors, so this should no longer be necessary.