Adding product demand constraints and update model docs

[kavithag17]

Added min/max constraints for the product demands at centers in build.jl
Updated problem.md to document the changes

[iSoron]

Hi Kavitha, thanks for working on this.

1. These demand constraints looks unnatural to me. I expected them to be per-center, not system-wide. It's hard for me to think of a realistic case where over-serving one location could actually compensate under-serving another location. My impression is that we are simply trying to artificially steer the optimal solution towards something we believe should happen (e.g., we should produce exactly a given amount, regardless of who consumes it) instead of running a model with realistic economic data/assumptions and seeing what actually happens.
2. Could you please also update the automated tests and the input data format?