Fix season watched/unwatched

[JonnyWong16]

Description

Fixes the filter for Season.watched() and Season.unwatched() to use viewCount to return the correct list of episodes. Also added tests for those two methods.

Fixes #677

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable