Multitask Solving, Hyper-parameter Tuning, Saving and Loading Model

Update

• Add save and load model functionalities in mealpy.utils.io module.
• Add object that hold global/current worst solution in history object
• Add method create_pop_group() in Optimizer class
• Add method before_initialization() in Optimizer class
• Refactor initialization() and after_initialization() in Optimizer class
• Remove before_evolve(), after_evolve(), and levy_flight() in Optimizer class
• Convert termination_start() and termination_end() to check_termination() in Optimizer class
• Remove boundary.py in utils
• Add set_parameters() and get_parameters() in all optimizers
• Update new Problem class, move problem parameter from Optimizer to solve() function.
• Fix bug printing same entry multiple times in logger.
• Fix bug exit() in Optimizer and utils package.
• Update new Termination class, move termination parameter from Optimizer to solve() function.
• Add Multitask class that can run multiple optimizers on multiple problems with multiple trials.
• Refactor all optimizers.
• Add Tuner class that can help tuning hyper-parameters of optimizer.
• Add examples how to build new optimizer.
• Add examples for Multitask and Tuner class.
• Update documents, examples, tests