Solver instance limit and instance cost

[parasj]

Solver now integrates a working instance limit and instance cost.

$ skylark solver solve-throughput aws:sa-east-1 gcp:australia-southeast1-a 9 --max-instances 2
       choose_solver:33  | Gurobi not installed, using CoinOR instead.
   get_transfer_cost:65  | No transfer cost found for aws:ap-southeast-1 -> aws:ap-southeast-3, using max of ap-southeast-1
   get_transfer_cost:65  | No transfer cost found for aws:ap-southeast-3 -> aws:ap-south-1, using max of ap-southeast-3
      print_solution:239 | Total cost: $0.3013 (egress: $0.2992, instance: $0.0021)
      print_solution:242 | Total throughput: [9.0] Gbps
      print_solution:243 | Total runtime: 0.89s
      print_solution:245 | Instance regions: [aws:sa-east-1=2.0, aws:us-east-1=0.031, gcp:southamerica-east1-a=2.0, gcp:southamerica-west1-a=1.537]
      print_solution:246 | Flow matrix:
      print_solution:253 | 	aws:sa-east-1 -> aws:us-east-1: 0.10 Gbps with 1.6 connections, 0.7GB (link capacity = 3.96 Gbps)
      print_solution:253 | 	aws:sa-east-1 -> gcp:southamerica-east1-a: 4.24 Gbps with 59.6 connections, 30.2GB (link capacity = 4.55 Gbps)
      print_solution:253 | 	aws:sa-east-1 -> gcp:southamerica-west1-a: 4.66 Gbps with 66.7 connections, 33.1GB (link capacity = 4.47 Gbps)
      print_solution:253 | 	aws:us-east-1 -> gcp:australia-southeast1-a: 0.10 Gbps with 2.0 connections, 0.7GB (link capacity = 3.25 Gbps)
      print_solution:253 | 	gcp:southamerica-east1-a -> gcp:australia-southeast1-a: 4.24 Gbps with 128.0 connections, 30.2GB (link capacity = 2.12 Gbps)
      print_solution:253 | 	gcp:southamerica-west1-a -> gcp:australia-southeast1-a: 4.66 Gbps with 98.4 connections, 33.1GB (link capacity = 3.03 Gbps)
    solve_throughput:75  | Could not render graph: [Errno 2] No such file or directory: 'xdg-open'