Represents the "Dur" entry of the navigation node dictionary object.

It is an optional entry defined as part of the PDF 1.0 specification.

It contains a double representing a PDF numeric object.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 163, page 380.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 165, page 461.