Further IIS enhanement

[jajhall]

Enhancements to be added in due course

• If time-out occurs when reducing the IS, return the current IS
• Apply the strategies given by iis_strategy to a MIP
• Use the 16-bit to attempt to find an IIS of a MIP relaxation, rather than the MIP itself. The latter calculation is very expensive since it requires the solution of multiple MIPs.

Clearly if the relaxation of a MIP is infeasible, an IIS is found very much more cheaply. Indeed, it makes sense to test whether the relaxation is infeasible before attempting to find even an IS of a MIP.

[jajhall]

The infeasible MIPs

• check/instances/infeasible-mip0.mps
• check/instances/infeasible-mip1.mps

will be useful