Simplify field-function user interface

[wdhawkins]

From a user-perspective, setting up and using field functions is somewhat messy:

        field_interp = FieldFunctionInterpolationVolume()
        field_interp.SetOperationType("max")
        field_interp.SetLogicalVolume(monitor_volume)
        field_interp.AddFieldFunction(fflist[3])
        field_interp.Initialize()
        field_interp.Execute()

We should simplify this... maybe via post-processors?

[andrsd]

I think the current volume postprocessor code should be able to do this. I can try with my PR to see, if that's the case. I will report back on this...

[andrsd]

I was able to replace the above with something like this:

ffi = VolumePostprocessor(
        problem=phys,
        value_type="max",
        logical_volumes=[monitor_volume],
        group=3
)
ffi2.Execute()
maxval = ffi2.GetValue()[0][0]

using my #916 PR.