add public Interval class (had been forgotten)
Revert "add public Interval class"

This reverts commit 4992a846e022c3693864c3b8478d9892ba1621b6.
private class Interval: improve, add documentation
Merge branch 'develop' of git://github.com/t-oster/LibLaserCut into develop
Merge branch 'develop' of git://github.com/t-oster/LibLaserCut into develop
Merge branch 'develop' of git://github.com/t-oster/LibLaserCut into develop
Merge pull request #5 from mgmax/develop

private class Interval: improve, add documentation