Numerics: BigInt Header

[1010jms]

Created a header file for the BigInteger class that contains the definitions for the constructors as well as the getter, the conversion functions to Rational and Real, the basic arithmetic operators, and the comparison operators.

Created the initial cpp file that contains comments explaining which algorithms need to be implemented for each function (as well as a link to each algorithm) as well as which functions need to be tested and which functions still need to be figured out.

Created unit test file that successfully tests the BigInteger constructors.

[1010jms]

Update:

• Moved all of the function bodies in the header file to the cpp file, which is currently under construction
• Added a default constructor
• Added more comments for the constructors, arithmetic operators, and comparison operators
• Removed the cpp and unit-test file from this pull request. Will create dedicated pull requests for each file in the future.