Update format checking to clang-format version 18

[petercamps]

Motivation
It has become hard to find and install clang-format version 9. However, the previous attempt in pull request #227 for updating the clang-format version was flawed. Please disregard any information in that pull request.

Description
With this pull-request the SKIRT repository is updated to use clang-format version 18.1. This is the most recent version that can be easily installed on Ubuntu 24.04 and with some effort on macOS as well.

Specifically:

• The formatSourceCode.sh shell script and the GitHub format checking workflow now use clang-format version 18.1.
• The formatSourceCode.sh shell script now performs the formatting on multiple cores in parallel.
• The Github format checking workflow now fails in case clang-format fails (e.g. because of an unsupported option).
• The SKIRT source code is updated to the slightly different formatting results.

Action required for developers
Developers who wish to contribute to the repository should install clang-format version 18.1 on their system and include its path in the PATH environment variable.

• On Ubuntu, this should be easy using the apt package manager.
• On macOS, the tool must be built from source code using an install tool such as port or brew. The clang-format tool included with Xcode is not lined up with official LLVM versions.