Resolving (parts of the) issue #381

[MatthiasHeinz]

Changes regarding issue #381:

• Fixing the wording in the error message.
• Changing the datatype from int and long to BigInteger.
• Addressing the performance issue by removing unnecessary multiplications and divisions.
• I did not remove the checks for num >= k >= 0, because there should be some sanity checks for the general computation of binomial coefficients in a reference implementation.

Checks before pull request:

• [x] I have performed a self-review of my code
• [x] My code follows the style guidelines of this project
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Comments in areas I changed are up to date
• [x] I have added comments to hard-to-understand areas of my code
• [x] I have made corresponding changes to the README.md (Remark: I've checked README.md, but no changes were necessary.)

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (52bf864) 95.75% compared to head (9dc76e8) 95.76%.

:exclamation: Current head 9dc76e8 differs from pull request most recent head d5a415c. Consider uploading reports for the commit d5a415c to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #392 +/- ## ======================================= Coverage 95.75% 95.76% ======================================= Files 218 218 Lines 8887 8898 +11 ======================================= + Hits 8510 8521 +11 Misses 377 377 ``` | [Impacted Files](https://codecov.io/gh/TheAlgorithms/C-Sharp/pull/392?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [Algorithms/Numeric/BinomialCoefficient.cs](https://codecov.io/gh/TheAlgorithms/C-Sharp/pull/392?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-QWxnb3JpdGhtcy9OdW1lcmljL0Jpbm9taWFsQ29lZmZpY2llbnQuY3M=) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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[MatthiasHeinz]

I'm time in time again confused as to why git thinks merging more than the desired commit d5a415cc992143da1e27ef92aadb56526958f6da would be a good idea... How does one fix this issue again?