harmonic spline improved for non continuous data


git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@1874 29e17f8d-cf37-471e-a288-72cc03a76eda
harmonic mean spline improved


git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@1875 29e17f8d-cf37-471e-a288-72cc03a76eda