Relative distance of other contexts in stencil callback

[Urs Liska]

Hi all,

is it possible from a stencil callback to retrieve the relative Y distance from 
the grob's staff to another staff given the other staff's name?

We want to create frames spanning multiple staves without having to do so in 
separate slices (see https://github.com/openlilylib/analysis/wiki/Frames for 
the general description and https://github.com/openlilylib/analysis/issues/3 
for the context of the current discussion).

If I know the relative position of another staff I can adjust the Y-Extent of 
the frame to include the other staff.

Any suggestions?
Thanks
Urs