Adding Vehicle Routing and Bin Packing Problems as well as TreeSizeEstimate as a reward function.

[ndrwnaguib]

Pull request checklist

• [ ] I have opened an issue to discuss the proposed changes: Fix #XX.
• [ ] I have modified/added tests to cover the new changes/features.
• [ ] I have modified/added the documentation to cover the new changes/features.
• [ ] I have ran the tests, checks, and code formatters.

Proposed implementation

I believe this is more of an extension to the currently supported instances, which are the vehicle routing and the bin packing problems (https://github.com/ds4dm/ecole/discussions/287). The latter can either be used independently or to estimate the number of minimum vehicles in the former.

I also added a new reward function, the tree size estimate, as reported by SCIP using https://github.com/scipopt/scip/blob/7786715f8b949d41febdad414615cec8af465173/src/scip/event_estim.c#L3017

Since the new loaders are not quite the same as the others, I thought I'd wait to hear your feedback before writing the tests, the new loaders mainly expect files as input either from http://vrp.galgos.inf.puc-rio.br/index.php/en/ for the Vehicle Routing Problem or http://people.brunel.ac.uk/~mastjjb/jeb/orlib/files/ for Bin Packing (or possibly files with the same format)