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.

etc/codeTemplates/dynamicCode/codedFvOptionTemplate.C

@@ -131,7 +131,7 @@ ${typeName}FvOption${SourceType}::
 void ${typeName}FvOption${SourceType}::correct
 (
     GeometricField<${TemplateType}, fvPatchField, volMesh>& fld
-)
+) const
 {
     if (${verbose:-false})
     {
@@ -148,7 +148,7 @@ void ${typeName}FvOption${SourceType}::addSup
 (
     fvMatrix<${TemplateType}>& eqn,
     const label fieldi
-)
+) const
 {
     if (${verbose:-false})
     {
@@ -166,7 +166,7 @@ void ${typeName}FvOption${SourceType}::addSup
     const volScalarField& rho,
     fvMatrix<${TemplateType}>& eqn,
     const label fieldi
-)
+) const
 {
     if (${verbose:-false})
     {
@@ -183,7 +183,7 @@ void ${typeName}FvOption${SourceType}::constrain
 (
     fvMatrix<${TemplateType}>& eqn,
     const label fieldi
-)
+) const
 {
     if (${verbose:-false})
     {