Mip

- Hopefully fixed a bug in successive MIP reoptimizations
- Lazy constraints support (still being documented but working !)

small fix for gurobi: do not call cut generator in some conditions

Fix for row prices in CBC and fix for comparisons of LinExpr objects. Thanks to spoorendonk .

- cut generators can now also add lazy constraints (remove integer solutions), initially on gurobi only.
- violation property in linear expression can check how much a solution violates the current solution

Fix for gurobi dll path on windows

Fix for loading gurobi using GUROBI_HOME environment variable