Fixed calculation of interpolated value at upper limit

A typo caused incorrect results for interpolated values at or beyond the
upper limit of the interpolation range. The unit tests were adjusted to
reflect this fix.
Merge pull request #16 from mkoennecke/bugfix/kernel-interpolation-edgecase

Fixed calculation of interpolated value at upper limit
Merge branch 'feature/9222_interpolation_defect' into develop