Refs #1465: Use proper linear extrapolation (instead of bilinear extrapolation) for bivariate Akima splines if only one input is to be extrapolated (and the other is to be interpolated)

git-svn-id: https://svn.modelica.org/projects/Modelica/trunk@7512 7ce873d0-865f-4ce7-a662-4bb36ea78beb