[Symphony] request: add a callback for user interruption

[ycollet at freesurf.fr]

Hello,

I am (still) working on an interface between Symphony and Scilab.
Some resolution can be a little bit long and adding a callback to allow the
user to interrupt nicely the computation can be a good thing.
For example: under scilab, a ctrl+c is catched via a boolean variable and
with a symphony callback, this value will interrupt the search.

YC