Reverted setFileName patch from Laurent for now, as it breaks the workflow for solvers

(which use the filename to guess which data file to read). Laurent's patch would set 
the filename to the last merged file, whereas we currently want the file name to be 
set to the projet file, i.e., the first merged file.

We should maybe think a bit harder about how to deal with this (maybe each Merge can 
set the fileName, but OpenFile takes precedence?)