Feature: Non Ambiguous names for days

[ramon-astudillo]

Using day0, day1, etc is ambiguous and error prone since we change the order often. Also it is difficult to relate exercises with tests. I propose the following:

• Use clearer naming scheme for each chapter e.g. linear_classifiers.
• Keep the name in references in the guide
• Use clearer naming scheme for each exercise e.g. backpropagation_numpy
• Keep the name in references in the guide

[ramon-astudillo]

Keep the name in references in the guide
Keep the name in references in the guide

This will be tackled with once we adjust the guide to the new version. Rest achieved.