v2.1.1

Update

• Replace all .copy() operator by deepcopy() operator in module copy. Because shallow copy causing the problem with
  nested list inside list. Especially when copying population with nested of list position inside agent.
• Add the Knapsack Problem example: examples/applications/discrete-problems/knapsack-problem.py
• Add the Linear Regression example with Pytorch: examples/applications/pytorch/linear_regression.py
• Add tutorial videos "How to use Mealpy library" to README.md