Completed extra credit 6

Implements:

• #1
• #2
• #3
• #4
• #5
• #6
• #7
• #8
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• #13
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• #17
• #18

All unit test pass.

Unit test compiles with 65 warnings in CalculatorBrainTests.swift, all related to the depreciated variables result, description and resultIsPending.