function object properties can only handle a single output

the sampled surfaces function object can contain several surfaces, but when it only has a single property to save the information. Eg,

Input:

plane0
{
    type            surfaces;
...
    surfaces
    (
        planeA { ... }
        planeB { ... }
    );

uniform/functionObjects/functionObjectProperties

plane0
{
    U
    {
        file "<case>/postProcessing/plane0/0.03/U_planeB.vtk";
    }
}

possibly relevant for #1091 (closed) @andy