[WIP] Add support for starting node

[fillipe-gsm]

Description

The available solvers provide solutions that always start at 0, with the exception of heuristic solvers solve_tsp_local_search and solve_tsp_simulated_annealing in case the user provides an initial solution x0, in which case it will respect the initial point of x0.

Adding support for any starting node seems to be a typical desired functionality as per the discussion in #21 and in #32.

Work to be done

• [x] Raise exception if starting node is larger than the number of nodes
• [ ] Check for mismatches between starting node and x0 if provided.
• [-] Add support for exact solvers
• [-] Add support for heuristic solvers.

Support for @luanleonardo's solvers will much likely require his help.