VoF: localize reading of alpha controls

src/finiteVolume/cfdTools/general/include/alphaControls.H

+const dictionary& alphaControls = mesh.solverDict(alpha1.name());
+label nAlphaCorr(readLabel(alphaControls.lookup("nAlphaCorr")));
+label nAlphaSubCycles(readLabel(alphaControls.lookup("nAlphaSubCycles")));
+Switch MULESCorr(alphaControls.lookup("MULESCorr"));
+
+if (nAlphaSubCycles > 1 && pimple.nCorrPIMPLE() != 1)
+{
+    FatalErrorIn(args.executable())
+        << "Sub-cycling alpha is only allowed for PISO operation, "
+           "i.e. when the number of outer-correctors = 1"
+        << exit(FatalError);
+}