Added Doxygen documentation for all files

[FloatingFowl]

All class (public) members have been documented (including the operators) along with the other helper/utility functions present in include/functions. Opening docs/html/index.html in a browser will give the documentation.

To regenerate the documentation, run cd docs; doxygen Doxyfile from the root directory after installing Doxygen.

Currently the documentation must be updated manually. The next step can be to integrate it with Travis.CI and possibly link to the documentation from the repo website.

Closes #4.

[codecov]

Codecov Report

Merging #28 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #28   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          13     13           
  Lines         536    536           
=====================================
  Hits          536    536

Impacted Files	Coverage Δ
include/operators/relational.hpp	100% <ø> (ø)	⬆️
include/operators/io_stream.hpp	100% <ø> (ø)	⬆️
include/functions/random.hpp	100% <ø> (ø)	⬆️
include/operators/increment_decrement.hpp	100% <ø> (ø)	⬆️
include/operators/binary_arithmetic.hpp	100% <ø> (ø)	⬆️
include/BigInt.hpp	100% <ø> (ø)	⬆️
include/functions/conversion.hpp	100% <ø> (ø)	⬆️
include/functions/math.hpp	100% <ø> (ø)	⬆️
include/operators/unary_arithmetic.hpp	100% <ø> (ø)	⬆️
include/operators/assignment.hpp	100% <ø> (ø)	⬆️
... and 3 more

---

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[faheel]

@FloatingFowl Thanks for adding Doxygen documentation! I think that only the Doxyfile should be committed, as the rest of the documentation files can be generated right before they're published after a successful CI build as you suggested.

[FloatingFowl]

@faheel The doc files have been removed

[faheel]

I'm quite busy currently so I'll only be able to review the updated documentation in detail by the end of next week (May 11).

[faheel]

@FloatingFowl Thanks a lot for your work on this! I've been working on other projects and had forgotten about this PR, so I apologize for merging it after such a long time. Thank you for your patience 🙂

[faheel]

I've merged this in the docs branch for now, which will be merged into master after a few more changes (that are being done in PR #50).