[WIP] Better exit file handling

[OndrejMarsalek]

Exit file is removed once used. Code structure is improved to remove code duplication - only Softexit performs checking.

As a bonus, we now also check for the file EXIT_<step number> to be able to cut a given run short at a specific step. This should be documented.

Some issues possibly remain, more testing needed - do not merge yet.

[ceriottm]

Changed target to master

[ceriottm]

@OndrejMarsalek as you see I'm doing some spring cleaning. Any chance this can be merged soon?

[mahrossi]

@OndrejMarsalek is this ready to merge?

[ceriottm]

Can this be merged?