Add on_event callback to solve() method

[tlambert03]

This pull request adds an optional on_event callback parameter to the solve() method in the code. The on_event callback function will be called when the solver emits an event and should accept an event data dictionary.

It can be used to monitor progress of the solution. At the moment, due to the different APIs in SCIP and Gurobi, the approach will slightly depend on the backend. But I can also help with how to interpret the data that the callback is receiving (hint, it will always be a dict following the ilpy.EventData declaration)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.62%. Comparing base (a6ce446) to head (7c6dc0a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #87   +/-   ##
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  Coverage   93.61%   93.62%           
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  Files          32       32           
  Lines         799      800    +1     
=======================================
+ Hits          748      749    +1     
  Misses         51       51           

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[tlambert03]

@cmalinmayor, this is ready for review

[cmalinmayor]

Code looks great, and I tested with the plugin and it worked perfectly 😃 thanks!!