Add concrete use-cases for attribute modification

[simonbowly]

We're missing good documentation and examples for attribute modification. A concrete example is #79: modifying the right hand sides of an existing set of constraints.

This needs some background to explain all the things that can be done with attributes in gurobipy and how this translates to gurobipy-pandas, but some quick how-to's would be a good start:

• how do I modify the bounds of a series of variables?
• how do I modify the right hand sides of a series of linear constraints?
• how do I set a MIP start?