Refactored FindPeakBackground. Refs #9020.

1. Refactored and thus fixed some issues;
2. Added  more unit tests;
3. Change input property. Algorithm can work with 1 spectrum only.
Adapted to new algorithm property. Refs #9020.
Make unit test to remove all genrated workspaces. Refs #9020.
Enable algorithm to accept 1 spectrum ws w/o. specify. Refs #9020.
Merge remote branch 'origin/feature/9020_fix_findpeakbkgd'