Add GitHub Actions CI workflow

[Copilot]

Replaces Travis CI with GitHub Actions for continuous integration.

Changes

• .github/workflows/ci.yml: Matrix build across Linux (gcc/clang) and macOS (clang), installing dependencies (cmake, boost, gsl), building with make, running ctest. Minimal permissions (contents: read) enforced.

• .gitignore: Excludes CMake artifacts, generated Bessel tables (>100MB), and test binaries.

• README.md: Updates badge to point to GitHub Actions instead of Travis CI.

The workflow mirrors the existing .travis.yml configuration.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• esm.ubuntu.com
  • Triggering command: /usr/lib/apt/methods/https (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

Add a yml for github actions that builds and runs the tests in this repository.

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