Linear extrapolation (and other options) for B-spline transformation

Addendum to pull request #1050. See the discussion there.

This includes now four different variants of extrapolation: linear (default), B-spline, constant, and "identity" (which could be useful for RT correction, as a "no change" fall-back). See here for documentation: https://github.com/hendrikweisser/OpenMS/compare/bspline?expand=1#diff-968241692c492b937cebfec674d78624R160