ENH: support tuple (pair) indexing into FieldField

applications/test/FieldFields1/Make/files

applications/test/FieldFields1/Make/options

applications/test/FieldFields1/Test-FieldFields1.C

applications/test/FieldFields2/Make/files

applications/test/FieldFields2/Make/options

applications/test/FieldFields2/Test-FieldFields2.C

applications/test/tensorFieldFields1/Make/files

applications/test/tensorFields1/Test-tensorFields1.C

src/OpenFOAM/fields/FieldFields/FieldField/FieldField.C

src/OpenFOAM/fields/FieldFields/FieldField/FieldField.H

...OpenFOAM/fields/FieldFields/oneFieldField/oneFieldField.H

...enFOAM/fields/FieldFields/zeroFieldField/zeroFieldField.H

src/OpenFOAM/fields/Fields/oneField/oneField.H

src/OpenFOAM/fields/Fields/zeroField/zeroField.H

src/OpenFOAM/primitives/tuples/Pair.H

src/OpenFOAM/primitives/tuples/Tuple2.H