NF: add to heuristic support of custom "post-processing" of entire set of files

[yarikoptic]

There is now a custom_callable support in the heuristic which is called for EVERY sequence during conversion. We would need something similar but working AFTER the loop finishes, so we could do final tune ups. (probably within convert function).

how should it be named? custom_callable_finalize

needed to possibly figure out associations for fmap files to corresponding func files, if we allow for such heuristic