Update user guide

Update main class

Merge branch 'branch-A-Personality'

Merge branch 'branch-A-BetterGui'

Merge branch 'branch-BCD-Extension'

Refactor findTasks() to use streams

Add assertions

There are no assertions in the program.

Absence of assertions makes it more difficult to detect mistake in code.

Add assertions in various points of the code to verify our assumptions.

Using assertions help to address the application's correctness.

Improve code quality

There are several areas where code does not follow best practices.

They can be improved to enhance code readability.

Refactor of several files. Adhere to practices such as SLAP, avoiding
deep nesting, complicated expressions and magic numbers.

Add assertions

There are no assertions in the program.

Absence of assertions makes it more difficult to detect mistake in code.

Add assertions in various points of the code to verify our assumptions.

Using assertions help to address the application's correctness.

Merge branch 'branch-Level-10'