TriggerJobOpts.runHLT: add support for postExecs

The only remaining use-case for the deprecated legacy "Modifiers" in CA-mode is the tweaking of the final ComponentAccumulator for special use-cases.

athenaHLT already supports this via the -C command line option. For athena we implement this via an additional --postExec command line option in runHLT.py. A new PostExec.py module is added that should host this code. For the moment the only use-case is the forceConditions postExec used to prepare preloaded SMKs for testing at P1 and can be used as follows:

athenaHLT.py -C 'from TriggerJobOpts import PostExec; PostExec.forceConditions(440499,470)' ...

Note that we no longer provide access to run and LB number (to avoid backdoors to run-dependent configs) so those have to be specified explicitly based on the input file.

For proper functioning requires !65197 (merged) (but can be merged independently).

cc @smh @tbold @fpastore @openc @wiedenma @mark and relates to ATR-26582