Hh add extra oo challenge

Added another OO challenge for those who are struggling with it.
Checking for equality with floats can sometimes be problematic because they are stored as approximations. You might want to use the `be_within` matcher: https://relishapp.com/rspec/rspec-expectations/v/2-5/docs/built-in-matchers/be-within-matcher

```ruby
expect(@assignment.average_grade).to be_within(0.001).of(93.7)
```