ENH: expose dictionary relativeName() method

src/OpenFOAM/db/dictionary/dictionary.C

src/OpenFOAM/db/dictionary/dictionary.H

src/OpenFOAM/db/dictionary/dictionaryEntry/dictionaryEntry.C

src/OpenFOAM/db/dictionary/dictionaryEntry/dictionaryEntry.H

src/OpenFOAM/db/dictionary/dictionaryTemplates.C

src/OpenFOAM/db/dictionary/entry/entry.H

src/OpenFOAM/db/dictionary/primitiveEntry/primitiveEntry.H