Add data preprocessing functions, model testing code, and refactor existing code

[Qubi0-0]

This pull request includes the following changes:

• Added new dependencies to requirements.txt

• Added data preprocessing functions for job title encoding and data normalization

• Added model testing code for K-Nearest Neighbors regression

• Fixed imports

• Updated model_testing.ipynb with Linear Regression and Decision Tree Regression models

• Added mean_sal_by_st_code column and fixed duplicate column name

• Changed outliers and updated saving file

• Added plots for better visualization

• Concluded more tests

• Improved plot

• Refactored data preprocessing and EDA code

• Added bar plot for work types

• Refactored code to improve performance and readability

• Added SVR and RFR models

• Added comments (the models still suck)

The pull request also includes the following file changes:

• Added pandas, ipykernel, jupyter, matplotlib, sklearn, os, and numpy to requirements.txt