- 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)