periodic spline again


git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@1907 29e17f8d-cf37-471e-a288-72cc03a76eda
include fixed


git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@1909 29e17f8d-cf37-471e-a288-72cc03a76eda