Gurobi FLK license cannot be used

**Describe the bug**
Gurobi offers a licensing method called FLK (Flexible Licensing Key). Previously this was known as ISV. This license requires parameters to be set on an empty Gurobi environment. The current implementation of the Gurobi interface for Python-MIP won't work for users with FLK keys.

**To Reproduce**
This can only be produced with a Gurobi FLK license. Unfortunately these are not openly distributed.

**Additional context**
A PR from Gurobi will be opened shortly. The associated documentation for using the proposed functionality will be distributed by Gurobi on a need-to-know basis.