Aims to let the InputMaker in empty Steps auto-configure how it merges its inputs in empty steps.
In steps where there is such thing as a most-recent feature (i.e. empty steps which are appended on to pad the end of a leg), we should merge using the features to test equality.
In steps where there is no feature yet (i.e. empty steps which are prepended on to pad the start of a leg), we should merge using the only thing we can be 100% is accessible which is the "initiaRoI" link added by the HLT seeding algorithm.
Crashed horribly when I tried to test myself. Requires re-compilation of all packages which use the changed public headers. Leaving the runtime tests to the CI for now.
Round 2 updates:
Still expecting runtime to fail, but with more helpful error messages.