C++ unit testing

[petrovicboban]

I did some research on which unit testing framework to use, and it looks like Catch seems reasonable choice in this moment. There is PR #112 on it.

https://www.reddit.com/r/cpp/comments/4e9afx/most_popular_c_unit_testing_frameworks/

If you have other ideas, please make proposal.

@edwardwliu @theo-s

[edwardwliu]

The three I would consider are:

1. doctest
2. Catch2
3. GoogleTest

The Forestry project is relatively small and we have limited C++ logic to implement. While the logic may be refactored, there will only be so many number of lines in C++ that will be added. In addition, the tests will likely remain relatively consistent and stable. Therefore, I'd prioritize a lightweight fast testing framework. As such, I'd suggest choosing between doctest and Catch2.

Doctest appears to be the simplest and fastest testing framework so that would be initial instinct. However, I see that Catch2 has existed for longer, has more support, and is updated more frequently. Let's stick with Catch2 for now and if any issues arise, we can consider an alternative.