Issue 129: Return primes list as read-only.

Alter GetAll to return a read-only IList of the prime numbers.

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[X] The new code follows the contribution guidelines
[X] I have performed a self-review of my own code
[X] I have commented my code, particularly in hard-to-understand areas
[X] I have made corresponding changes to the documentation
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[X] I have added tests to ensure that prove my fix is effective or that my feature works
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aalhour / C-Sharp-Algorithms