Add a stub past tense class. Needs conjugation rules, but will ensure we pass some of the failing tenses (for tense lists, etc.)
user
```ruby
# encoding: UTF-8

class NorskPastTense < NorskTense
  @keys = "past", "fortid"

  def self.specific_conjugation(verb)
    "STUB - CONJUGATION RULES NEEDED"
  end
 end
```
assistant