Odd - why is FunctionInterface necessary?  It really should be inherited further down the chain.  @permcody can you explain why we don't do this down in the base class (or at least in the intermediate bases like SideUserObject)??

If you look at the inheritance diagram for FunctionInterface you can see the weirdness.  It's inherited by all base classes... and then a bunch of concrete UserObject classes: http://www.mooseframework.org/docs/doxygen/moose/classFunctionInterface.html

@WilkAndy This isn't your fault - you're doing the right thing and this won't keep us from merging this - but it has pointed out a possible issue with our inheritance hierarchy...