Refactor and clean CMake files

[FergusonAJ]

This PR contains a collection of incremental improvements to our CMake infrastructure:

• Automatically find .cpp files to compile
  • For main application executables, these will be any .cpp files in /source/
  • For unit tests, these will be any .cpp files in that particular unit test folder (e.g., /tests/unit/core/*.cpp )
• Add a TEST build type that will build only the tests
  • Extensible if we want ever want to turn off tests for certain build types
• Streamlining of CMake files in the test directories
• Add support for CTest
  • This builds off of Ctest for testing #38 from @amantham20
• Add CMake files for other unit test directories (Worlds, Agents, and Interfaces)
• Clean and comment main CMake files
• Updates made to HowToCompile and HowToTest documentation
• Source files handled via .cmake files with the same name as the .cpp
  • e.g., simple_main.cpp will load a simple_main.cmake which can specify source files
  • This only applies if the file exists
  • For an example, see source/example.cmake
  • This should handle situations like the PR by @rikipls (Adding PathAgent, support for running Catch2 tests with CMake mdkdoc15/group_1_cse_491#3)

Closes #38

[amantham20]

you might have to explain to me how the linking with all the hpp files work and how to create a new cofig/executable but except for that and the mentioned changes everything works good!