Enhance complete_analysis with model saving and add model loading functionality

[WladRamos]

This Pull Request introduces two key improvements:

1. Model Saving in complete_analysis:

   • The complete_analysis function has been enhanced with the ability to save the trained model to a .pkl file. This allows users to preserve models after training for later use without retraining.
   • The user can specify the filename for saving the model or leave it blank to use a default name with a timestamp.
   • The model is saved conditionally, based on the save_model parameter.

2. Function to Load a Saved Model:

   • A new function load_model was added to load a model from a .pkl file. This allows users to reload the saved model and continue making predictions or further analysis.

3. Unit Tests:

   • Added tests to ensure that the model is saved correctly and can be loaded without errors.
   • Verified that the loaded model retains its functionality and can make predictions with the test data.
   • Ensured proper cleanup of the saved model file after tests.