BUG: Fix typo in _AnnotationBase

Value of "72" switched to "12" in d5a1a3dd36f7daaf8b7e2ec5a73fa5fd4df3e026
Merge pull request #1620 from tonysyu/annotation-bug

Fix bug in _AnnotationBase
Added TriFinder and TrapezoidMapTriFinder classes.
Added LinearTriInterpolator class.
Improved PEP7 compliance.