harmonic spline - given up to find a good solution for non increasing x


git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@1888 29e17f8d-cf37-471e-a288-72cc03a76eda
qwt_spline code cleanup


git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@1889 29e17f8d-cf37-471e-a288-72cc03a76eda