Open source alternative for Gurobi

I'd like to consider for netflow to use open source alternative for Gurobi as a solver. It seems that HiGHS will do the job without changing anything for pre-existing Gurobi path. So far, I'm only thinking to use this for the target uploader and applying some modification of netflow v3.4. Then, I'll check if it works on the master together with other updates on the uploader. 

Here is the branch for it: https://github.com/Subaru-PFS/ets_fiberalloc/tree/tickets/FIBERALLOC-62

If you think that it's better to split the new HighsProblem class into a new file, please let me know.