v1.4.2 : ensure run tree is written to output event file

If a processor was calling getHistoDirectory() during processing (and not just in onProcessStart()), then the run tree would end up in the output histogram file and not the output event file. This simply adds the extra directory movement necessary to inform ROOT that we want the run tree in the output event file.

What's Changed

• need to enter the output event file before creating and writing by @tomeichlersmith in #93

Full Changelog: v1.4.1...v1.4.2