add threshold_lo_ & threshold_hi_

[Youhaojen]

fix the issue "If the system has exploded, unphysical structures may be saved since no upper bound is set on the uncertainty threshold."

Summary
The active learning workflow can be used with active <interval> <has_velocity> <has_force> <threshold_lo> <threshold_hi>

Modification
Rename threshold_ to threshold_lo_, and add threshold_hi

Others

[brucefan1983]

Thanks for the contribution. It seems you separated changes into two pull requests? Could you complete one and delete the other?