Add sample problem including AllEqual constraint

[jardinetsouffleton]

Problem statement (idea)

A company wants to organize a team-building event for its employees, who are divided into three departments. The company can choose from a list of five different activities. Each department ranks the activities based on their preferences. The company wants to select an activity that minimizes the total "dissatisfaction," which is the sum of the absolute differences between the departments' rankings and the final activity.

Mathematically, this goes like this:

Let $N$ be the number of departments and $M$ be the number of activities. Let $p_{i,j}$ represent the preference ranking of activity $j$ by department $i$. The preference rankings are given and non-negative. Let $x_j$ be a binary variable that takes the value 1 if activity $j$ is selected and 0 otherwise.

The objective is to minimize the total dissatisfaction:
min $\sum\limits_{i=1}^N \sum\limits_{j=1}^M p_{i,j} * x_j$

Only one activity can be selected
$\sum\limits_{j=1}^{M} x_{j} = 1$

Binary selection on the decision variables
$x_j ∈ {0, 1} \forall j = 1, ..., M$

Then, the goal of this issue is to create an example where the AllEqual constraint is used to solve the sample problem.