Use assertions to ensure that dates make sense and change the implementation of operators <= and >=, increasing the readability

- Used a docker image with a more recent version of Debian
- Enabled colours for gcc and clang
- Updated clang-tidy from 4.0 to 7.0
- Enabled some diagnostic checks for clang-tidy, and disabled others that gave false positives
- Made clang-tidy read system header files, while ignoring their errors

Clang-tidy complaints, so the if-else structure of the implementation of the operators changed. The code is a little bit ugly so it should be evaluated