locations of interface descriptions

[Mark Polesky]

I see that interfaces come to the IR from at least three
different types of places:

1) lily/[grobname].cc              (e.g. accidental)
2) lily/[grobname]-interface.cc    (e.g. align)
3) scm/define-grob-interfaces.scm  (e.g. ambitus)

Is there a system to this organization?  Is it optimal?
It's confusing to me.  If it's just that I don't understand
it, and the system is well-organized, could we then add a
one-line comment at the top of the scm file to let less
experienced developers like me get some bearings?

Thanks.
- Mark