More quality-of-life improvements: Exp/Eval/Gen limits, and soft limits

[guicho271828]

This commit adds separate limits for expansion / evaluaiton / generations, and soft limits.

Soft limit addresses the limitation of hard limits (e.g. max_time) that one limit interferes another. For example, when max_eval = 100k and max_time = 5 sec, majority of runs are cut off before reaching 100k evaluations, and the result is not an appropriate representation of running 100k evaluations. To measure the evaluations and time separately, therefore one would run two separate experiments, which is a waste of compute for instances solved by both. Instead of running them separately, this option allows one to run the union of the two configurations which can be later filtered to produce distinct tables/figures.

(Tbh I was surprized that FD does not have this feature yet.)

[guicho271828]

If you are eager to add a class for representing the limits, I can do that too.