fvOptions: Changed the source, constraint and correct functions to const

Most fvOptions change the state of the fields and equations they are applied to
but do not change internal state so it makes more sense that the interface is
const, consistent with MeshObjects.  For the few fvOptions which do maintain a
changing state the member data is now mutable.

tutorials/multiphase/interFoam/laminar/damBreakWithObstacle/constant/fvOptions

@@ -0,0 +1,31 @@
+/*--------------------------------*- C++ -*----------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     | Website:  https://openfoam.org
+    \\  /    A nd           | Version:  dev
+     \\/     M anipulation  |
+\*---------------------------------------------------------------------------*/
+FoamFile
+{
+    version     2.0;
+    format      ascii;
+    class       dictionary;
+    location    "constant";
+    object      fvOptions;
+}
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+inletTurbulence
+{
+    type            vectorFixedValueConstraint;
+
+    selectionMode   all;
+
+    fieldValues
+    {
+        U           (0 0 0);
+    }
+}
+
+
+// ************************************************************************* //