bezier code moved to qwt_bezier.[h|cpp], natural spline code modified


git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@1858 29e17f8d-cf37-471e-a288-72cc03a76eda
generalized spline interpolation added


git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@1859 29e17f8d-cf37-471e-a288-72cc03a76eda
spline polynon to bezier path translation added


git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@1860 29e17f8d-cf37-471e-a288-72cc03a76eda