periodic spline again


git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@1924 29e17f8d-cf37-471e-a288-72cc03a76eda
comments aboit how substitution for parametric splines works


git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@1925 29e17f8d-cf37-471e-a288-72cc03a76eda