Disable saving output on both & single mode

user request:

only populate the Filename box with a workspace name if a single detector
has been reduced (front/rear/main/hab) **or** if it is the MERGED
workspace. In the case of BOTH, leave it empty and force the User to
select a Save algorithm.

re #6929
Merge branch 'feature/6929_use_sancas_v1_1' into develop