[dune-pdelab] Add data to grid boundary

[Dion Häfner]

Dear PDElab developers,

in my module, I want to store some boolean value for each boundary face,
which should be inherited by child elements when using adaptive grid
refinement. I thought of implementing it using a float which is either
.0 or 1.0, interpolating it to the new grid using PDElab::interpolate,
and rounding to the nearest integer to get a binary value again.

The problem is, that I cannot calculate this value directly from
position, but only from the history of a cell. Is there a built-in way
to interpolate from discrete values instead of a function? And which
type of container could I use, so I can easily reference the value for
each boundary face?

Thanks in advance,
Dion