Refactor config file management
Config file management needs to be simplified:
- From command line, start Met.3D with something like "met3d --config=myconfigfile.cfg --datesets=grid1.cfg,grid2.cfg,traj1.cfg --session=mysession.cfg"
- Requires moving remaining config items from default_pipeline.cfg to default_frontend.cfg
- Requires cleanup of frontend.cfg: remove sessions and actors
- Make file name suffixes consistent (cfg vs conf vs config)