Termination callback and boolector_pop() about boolector

[mpreiner]

When we initially implemented termination support, we didn't think about incremental usage. As soon as the termination function returns true, we internally set a termination flag to true. We don't reset the flag. Having the termination callback per check-sat makes sense.
There are three options to add support for this:

• Add an option to enable reset after each check-sat call
• Add an additional reset flag to boolector_set_term to indicate whether the termination flag should be reset after check-sat
• Do not set a flag at all and only rely on the return value of the termination function

[aniemetz]

@mpreiner @andrewvaughanj do we plan to include this in #62? Is this already supported by #62?