add a util function: BaseScan.get_relative_file

Both tomwer and nabu have some code to retrieve the relative file for '[dataset]_nabu_processes.h5' or '[dataset]_tomwer_processes.h5'. We could group the code and simplify both code by having a function like

class TomoScanBase:

   def get_relative_file() -> str: # / get processing_file
      pass