Adds a GitHub action that runs a script that checks for O2-specific issues in the code.
The script can run standalone, taking paths of files as arguments.
When running in the GitHub action on the pull_request event, the script tests files modified by the PR and generates error messages as GitHub annotations which appear as comments on the relevant lines or at the top of the file in case of per-file tests.
When running in the GitHub action on the push event, the script tests files modified in the branch w.r.t. the main branch and does not produce annotations.
False positives should be rare but, if any, they can be silenced per line for most of the tests by adding a comment in a given format.

Implements the following tests.

Bad practice:

• included iostream
• importing names from the std namespace
• using directives in headers
• missing std:: prefix for common names from the std namespace
• unnecessary use of ROOT entities
• use of external pi
• adding/subtracting of 2 pi
• multiples/fractions of pi for existing equivalent constants
• use of TDatabasePDG
• use of hard-coded PDG codes
• unnecessary call of Mass() for a known PDG code
• non-O2 logging
• not using const refs in range-based for loops
• not using const refs in process function subscriptions
• usage of workflow options in defineDataProcessing

Documentation:

• mandatory documentation of C++ files

Naming conventions:

• functions
• variables
• macros
• constexpr constants
• namespaces
• defined types
• enumerators
• classes
• structs
• O2 columns
• O2 tables
• O2 workflows
• explicit task names
• C++ files
• Python files

PWG-HF specific conventions:

• PWGHF: swapped const auto
• PWGHF: names of structs and classes
• PWGHF: order of struct members
• PWGHF: names of workflow files
• PWGHF: names of configurables