Automatic tests and examples

[GPMueller]

Closes #72.

• add CLI flags
• discover and build tests and examples of the root project
• add ability to automatically run tests
• get the recursive versions running
• unit tests
• documentation

This is currently based on the feature-package branch of PR #98 and will be rebased onto master when PR #98 has been merged.

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This design has drawbacks:

Take as an example the following suggestion for how to specify tests:

name = "my_project"

[my_lib]
    # blaaaaaa
some_var = 12

[test]
[test.my_exe_test]
    dependencies = "my_lib"

• If no tests are specified, no test sources are searched for, i.e. no sources_exclude is necessary when unwanted sources are picked up in test discovery.
• specifying all tests (per project) under one node in the toml can help to keep things tidy (though for boost, this would be completely terrible)

two differences between the two approaches:

1. default search for sources: manually specify if tests are present vs. manually specify when all sources should be ingored
2. organization in the toml: tests in one single block vs. tests each attached to their respective targets

[codecov]

Codecov Report

Merging #102 into master will increase coverage by 0.20%.
The diff coverage is 73.68%.

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
+ Coverage   80.26%   80.47%   +0.20%     
==========================================
  Files          12       12              
  Lines        1059     1188     +129     
==========================================
+ Hits          850      956     +106     
- Misses        209      232      +23     

Impacted Files	Coverage Δ
clang_build/clang_build.py	65.54% <51.72%> (-3.80%)	⬇️
clang_build/project.py	73.86% <77.70%> (+3.91%)	⬆️
clang_build/dependency_tools.py	68.08% <88.88%> (+4.19%)	⬆️
clang_build/io_tools.py	100.00% <100.00%> (ø)
clang_build/target.py	88.92% <100.00%> (ø)
clang_build/single_source.py	89.23% <0.00%> (+21.53%)	⬆️

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