Passes down the list of input-to-output relations to the field callback.

Passes down the list of input-to-output relations to the field callback.
It allows this callback to actually use the input in a proper way (the first
parameter of the cb is the user input, which is improperly keyed).

Also make the getValue method public and static to simplify the computation
of output keys by the callbacks.

Signed-off-by: Vincent Zanotti <vincent.zanotti@m4x.org>