[CVaR]: Add CVaR term in the objective function

[datejada]

Description

CVaR term:

$$\lambda \cdot \left(\mu + \frac{1}{1-\alpha} \sum_{s \in S}{\pi_{s} \cdot \xi_{s}} \right)$$

• Add the CVaR term to the objective function here: https://github.com/TulipaEnergy/TulipaEnergyModel.jl/blob/main/src/objective.jl
• This term only goes in the objective function if lambda is greater than zero; otherwise, it is zero, or just don't add it.
• Add a test that checks that this term exists and it is correctly added when lambda > 0. Create a test with only one scenario and with two or more scenarios