additional patch expressions functions

as noted in EP1558 - the missing internalField(..) function is needed

expressions intentionally left out several more advanced functions, with the aim of reduced parser size and the assumption that very complicated expressions might be better served by dynamic coded equivalents.

However, internalField() and neightbourField() and snGrad() seem to be common enough to include.

@Prashant - may need to include more tests into the test suite to test full coverage etc.

mentioned in commit b6b4ab07

mentioned in commit a3d90ae9

closed