ENH: Added isAbsolute() function to fileName class

src/OpenFOAM/db/IOobject/IOobject.C

...db/dictionary/functionEntries/includeEntry/includeEntry.C

src/OpenFOAM/global/argList/argList.C

src/OpenFOAM/primitives/strings/fileName/fileName.C

src/OpenFOAM/primitives/strings/fileName/fileName.H