Add lifted successor generator based on enumerating all maximum cliques

[simon-stahlberg]

Add the lifted successor generator from the paper titled 'Simon Ståhlberg, Lifted Successor Generation by Maximum Clique Enumeration, ECAI 2023.'

I have tried to change as few files as possible while adhering to the coding style of the repository. However, there are a few notable changes:

• In src/search/[action_schema | object | structures].h, I have implemented comparison operators and hash functions.
• In src/search_engines/**.cc, the iteration over action schemas has been replaced. Instead, the successor generator is now provided with a list of action schemas. This change was made because some computations by the new lifted successor generator are shared among all action schemas. As far as I can tell, the performance impact of this change is not measurable. (The diff is a bit messy because of indentation.)

Let me know if any changes need to be made for this commit to be merged.

[DillonZChen]

Hi @abcorrea. Will the pull request be accepted any time soon?

[abcorrea]

Hi @DillonZChen,

thanks for reminding me of this. I was on medical leave when the pull-request happened, and it just slipped through my TODO list. I will rebase and merge it now.