Parser: better #codeStream

We have an issue with such OpenFOAM input code:

FoamFile
{
    version     2.0;
}

internalField   #codeStream
{
    codeInclude
    #{
        #include "fvCFD.H"
    #};

    codeOptions
    #{
        -I$(LIB_SRC)/finiteVolume/lnInclude \
        -I$(LIB_SRC)/meshTools/lnInclude
    #};
    codeLibs
    #{
        -lfiniteVolume \
        -lmeshTools
    #};
    code
    #{
        const IOdictionary& d = static_cast<const IOdictionary&>(dict);
	    const fvMesh& mesh = refCast<const fvMesh>(d.db());
        scalarField alpha_a(mesh.nCells(), 0);

        forAll(mesh.C(), i)
        {
            scalar y = mesh.C()[i].y();
            alpha_a[i] = 0.305*(1.0+tanh((12.5*0.006-y)/0.005));
	    /*if (y < 12.5*0.006)
	    {
		alpha_a[i] = 0.61;
	    }
	    else
	    {
		alpha_a[i] = 0;
	    }*/
        }
        alpha_a.writeEntry("", os);
    #};
};

We should be able to get/set the code with:

print(tree.children["internalField"]["code"])
tree.children["internalField"]["code"] = "..."

Or even maybe (because we would have a CodeStream node, which would just be a special Dict):

print(tree.children["internalField"].code)
tree.children["internalField"].code = "..."

Note that I guess it should be very easy to add few lines to tests/test_parser.py::test_code_stream to make it fail by testing this behavior.