ENH: distinguish between compressible/incompressible coded source

- previously had codeAddSup used for both incompressible and
  compressible source terms. However, it was not actually possible to
  use it for compressible sources since any references to the 'rho'
  parameter would cause a compilation error for the incompressible case.

  Added 'codeAddSupRho' to distinguish the compressible case.
  User must supply one or both of them on input.

src/OpenFOAM/db/dynamicLibrary/dynamicCode/dynamicCodeContext.C

@@ -100,6 +100,12 @@ bool Foam::dynamicCodeContext::valid() const noexcept
 }
 
 
+const Foam::entry* Foam::dynamicCodeContext::findEntry(const word& key) const
+{
+    return this->dict().findEntry(key, keyType::LITERAL);
+}
+
+
 bool Foam::dynamicCodeContext::readEntry
 (
     const word& key,