[QC-1176] Display a warning for developers if a registered object is not mergeable

[justonedev1]

this is just a draft with an idea, how to do it compile time, I am not sure, that you like it. It does not build, it would need a bit more work. Otherwise I don't know how to do this compile time and it would need to be done run time via dynamic_cast

[knopers8]

For the record, we discussed in person, the agreed improvements are:

• move the type handling logic to Mergers
• find a way to avoid including all of the complete types that are checked (typically, the user will include what they need)
• fail the compilation if the type is not mergeable, but add another method like "startPublishingNotMergeable" to allow expert users to do what they want.

[justonedev1]

I think that I addressed everything that we talked about, but a bit different way. If you would prefer something else, I can do that.

[knopers8]

Thanks, just some details to sort out, but in general the idea is good.

Do I understand correctly that the inclusion order of ObjectManager.h and PostProcessingInterface.h should not matter because the template is put in place after both?

[knopers8]

thanks, it seems all good now!

[justonedev1]

@wiechula Can you please take a look at this PR?