Add actual support for n-class MRs

[0152la]

Currently, we distinguish between what we call relations and generators. The functional difference is that relations can be emitted at the level of the metamorphic family chain, while generators can only be called within these relations MRs. Essentially, there is a depth level of 1 requirement in order to emit a generator MR.

We should replace this feature, and generalise in case of usage. Namely, 0-class MRs should be those allowed to be emitted at any point, while n-class MRs would require at least n placeholder calls before the MR can be selected for the respective family. This could force MRs with more placeholders to be emitted earlier, and leave other, more basic MRs, to be emitted lower in the MR recursive chain.