GitLab

Add a pareto parameter in the objectives
Add a _pareto_objectives_list in the units
Add a pareto_models attribute in model
Add a methods to calculate a pareto front in model
Add a plot_pareto2D method to plot a pareto front based on two quantities

In order to enable one to access to the values of the quantities,
calculated after the optimisation, by the model and not only by the units

[PuLP library version changed from >=1.6.10 to 2.1: changed imports in general.optimisation.model, as well as requirements.txt, setup.py and associated docs]

[removing LpVariableDict import in model since it is not used and has been removed from pulp in its last version]

[in set_operating_time_range(), [[str, str]] is not the default value but the default type]: "=" changed into ":"