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)

applications/utilities/preProcessing/setExprBoundaryFields/setExprBoundaryFields.C

@@ -33,8 +33,8 @@ Description
     Set boundary values using an expression
 
 Note
-    Based on funkySetBoundaryFields
-    Copyright 2006-2018 Bernhard Gschaider <bgschaid@hfd-research.com>
+    Based on funkySetBoundaryFields from
+    Bernhard Gschaider <bgschaid@hfd-research.com>
 
 \*---------------------------------------------------------------------------*/
 
@@ -248,8 +248,12 @@ int main(int argc, char *argv[])
 
                 expressions::patchExprDriver driver(currDict, mesh);
 
-                // Search on disc
-                driver.setSearchBehaviour(cacheFields, false, true);
+                // Search files only
+                driver.setSearchBehaviour
+                (
+                    expressions::exprDriver::SEARCH_FILES,
+                    cacheFields
+                );
 
                 driver.clearVariables();
                 driver.parse(expr);

applications/utilities/preProcessing/setExprFields/setExprFields.C

@@ -33,8 +33,8 @@ Description
     Set values on a selected set of cells/patch-faces via a dictionary.
 
 Note
-    Based on funkySetFields
-    Copyright 2006-2018 Bernhard Gschaider <bgschaid@hfd-research.com>
+    Based on funkySetFields from
+    Bernhard Gschaider <bgschaid@hfd-research.com>
 
 \*---------------------------------------------------------------------------*/
 
@@ -310,11 +310,9 @@ void evaluate
 
     Info<< endl;
 
-    expressions::volumeExprDriver driver
-    (
-        mesh,
-        ctrl.cacheVariables
-    );
+    expressions::volumeExprDriver driver(mesh);
+
+    driver.setCaching(ctrl.cacheVariables);
 
     driver.readDict(dict);