Update Quokka workflow model to support empty beam and cell file association propagtion

During the demo discussion on 19th Nov 2009, the Quokka instrument scientists have brought up the requirement of handling scenario where empty beam and empty cell are done in different experiment.

In this case, the reduction code should handle loading of file associate from different report (so called the master report), or from the previous experiment. The solution to this is to modify the existing experiment model so that for each configuration it will carry the empty beam and empty cell file names.

Case 1:
Scientist may reuse the existing configuration from previous run (which has empty beam and cell measurement completed), and wants the file association be carried on.

Case 2:
Scientist may use an existing report file that contains empty beam and cell information, and perform experiment so that the online reduction component knows where to fetch necessary data.

Case 3:
Scientist may specify a different master file for online and offline analysis.