Introduced opaque library from Kyle Markley and improved DateTimeRange class Opaque library allow safer manipulation of numeric values and gives tem a meaning. This should help to prevent stupid mistakes. A first usage in SciQLop is Seconds which are double but have a specific meaning, a DateTimeRange can only be multiplied by a double (-> zoom) but only seconds can be added to it (-> pan). Even a zoom could be an opaque type in the future: Range/Range->Zoom, Range*Zoom->Range, Zoom*Range->Error. DateTimeRange class has now many arithmetic operators implemented this will allow to implement zoom/pan operations in only one place and test them. Signed-off-by: Alexis Jeandet <alexis.jeandet@member.fsf.org>