rework nexus-file management
We could create a cache or create modified nexus file at some processes. The 'first' / raw nexus file should stay 'untouched'. Then we should process on other files. One file per process ? For example modifying image_key(s) should be done in a separate file.
In this case we should also provide a widget for 'overwritting the nexus file' or 'dumping / copying' current nexus file. This is the same for tomwer_processes.h5. We could have several when there is some 'branch' fork. And we could have file to dump processes of one branch only.