Extended ScopedVector of clear and of accessing particular scopes

[Tomaqa]

Extended ScopedVector to allow more operations and look more std. container-like.

Importantly, I added functions that allow accessing particular scopes, not only the entire vector, and not even necessarily the top scope. This allows traversing the elements in batches in a lazy fashion, as for example done with frames in the preprocessing pipeline.

Btw., AssertionStack could probably also be rebased on top of a ScopedVector, but it would not be that straightforward.

[blishko]

Can you link to a proposed usage of the new functionality?

[Tomaqa]

I will most likely use it for the new feature of decision preferences, where I want to keep track of the processed frames in simplifyFormulas, along with the preferences that are stored in a ScopedVector, because I will probably need to use the substituted formulas instead of the inserted ones and use their actual variables and clauses that are sent to the core solver.

It is still a WIP, I do not even have a PR for it yet. The current version does not interfere with the preprocessing, which may be the reason why it is not working at the moment :)

If the extension is too much, maybe we can make it a derived class?

[Tomaqa]

#873 currently requires this PR + #872