[Cgl] Specification of a cut generator

[fmargot at andrew.cmu.edu]

I have added a wiki page https://projects.coin-or.org/Cgl/wiki/infoSpm
with a proposal of specification of a cut generator in Cgl. The goal
is to have a uniform way to interact with all the generators in term of
parameters and data. Please edit and/or comment on it. This is just a
first draft, and I am quite flexible about most things. Due to the
growing number of generators, I feel the need of putting guidelines a little
bit more detailed in order to simplify somewhat the life of an average user.

Francois