05_sumAll - Add alternative solution using Gauss formula

[oomertaa]

Complete the following REQUIRED checkboxes:

• [ X] I have thoroughly read and understand The Odin Project Contributing Guide
• [ X] The title of this issue follows the location for request: brief description of request format, e.g. Exercises: Add exercise on XYZ

The following checkbox is OPTIONAL:

• [ X] I would like to be assigned this issue to work on it - Yes, but I don't know how i would be able to add to the official repo.

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1. Description of the Feature Request:

2. Acceptance Criteria:

3. Additional Information:

[thatblindgeye]

While this additional solution is helpful from a maths standpoint, I don't think it offers significant benefit from the intent of the exercise, which is to practice writing unit tests.