ENH: additional patch expressions (#2114)

- snGrad, internalField, neighbourField.
  Functional use as per swak: "... + internalField(T) ..."

ENH: additional volume/patch expressions

- deltaT()

STYLE: rename exprDriverWriter -> fvExprDriverWriter

- the original class name was a misnomer since it holds a reference
  to fvExprDriver

BUG: expression faceToPoint/pointToFace definitions were flipped

ENH: refactor expression hierarchy and code style

- handle TimeState reference at the top-level for simpler derivations

- unified internal search parameters (cruft)