Allow specifying objectives as maximization problem

[schmoelder]

Currently, all objective functions are expected to return a value that is to be minimized.
However, in reality, objectives are often to be maximized (e.g. a productivity).

This PR adds a new flag to the add_objectives method. If minimize is False, the result will automatically be multiplied with -1 internally to convert the maximization problem to a minimization problem.
For reporting, the original value is then used.

Moreover, for nonlinear constraints, the user can now specify a comparison_operator (default='le') which is used to compare the output of the nonlinear constraint function with the specified bounds.
In any case, the "true" value of the nonlinear constraint function, as well as the constraint violation are stored for reporting, where positive values indicate the constraint is not satisfied.

To do

pending #114

• Add minimize flag.
• Add flag for nonlinear constraints
• Update documentation (e.g. custom method for fractionation)