Stress that all Nups should be stateless w.r.t. gurobipy data structures

[rluce]

The simplicity of a pure data I/O interface like a function, or a class/method call implies that no Env, Model state can be made persistent across multiple invokations. Applications that require such state, say, for dynamic processes, are not in the scope of the NupStup, and should target gurobipy instead. This should be mentioned in the contributing guideline.

[simonbowly]

Note added, and existing mods converted