text: Title: mlfromscratch/supervised_learning/__init__.py has references to undefined python files.
  Body: Everything you need to know is in the title. ` mlfromscratch/supervised_learning/__init__.py` has references to undefined python files.
  
  To reproduce, run: "python demo.py", you get:
  
      ImportError: No module named linear_regression
  
  Because `supervisedLearning/__init__.py` has references to regression classes in files that don't exist.
  
  I was able to correct them in my fork and rebuild and all is well, just have to rename the "whatever_regression" to regression and do the setup again.
  
  Thinking it over, these may be intentional as an exercise for the user.  Good job.  :1st_place_medal: 