Improve documentation Model Generation

[gsilano]

Hi @chenyutao36! This is Giuseppe Silano, a post-doc researcher at the Czech Technical University in Prague. These days, my colleague (@hendrik-s-debruin) and I have been looking at your code, especially the Model_Generation.m script. The aim was to understand the improvements made to the script by the latest reference publications [1, 2]. So, we started commenting out the functions in the Model_Generation.m script trying to link to the papers. At the end of the day, we thought this work might be useful for others, too. Hence the reason behind this pull request.

We think It would be great if you could review them, fixing or improving misleading or unclear parts, and then update the code in your repository.

[1] Y. Chen, M. Bruschetta, E. Picotti, and A. Beghi, "MATMPC - A MATLAB Based Toolbox for Real-time Nonlinear Model Predictive Control," 2019 18th European Control Conference (ECC), 2019, pp. 3365-3370, doi: 10.23919/ECC.2019.8795788. [2] Y. Chen, M. Bruschetta, D. Cuccato, and A. Beghi, "An Adaptive Partial Sensitivity Updating Scheme for Fast Nonlinear Model Predictive Control," in IEEE Transactions on Automatic Control, vol. 64, no. 7, pp. 2712-2726, July 2019, doi: 10.1109/TAC.2018.2867916.