Update Go version to 1.27 and enable SIMD compilation

This commit updates the project to use Go version 1.27 for all build and testing processes. It also introduces support for experimental SIMD code compilation, enabling it by default while ensuring a portable scalar fallback is available. Documentation has been updated to reflect the new Go version and the use of the experimental SIMD package.

.github/workflows/obitest.yml

@@ -12,7 +12,7 @@ jobs:
     - name: Setup Go
       uses: actions/setup-go@v5
       with:
-        go-version: '1.23'
+        go-version: '1.27'
     - name: Checkout obitools4 project
       uses: actions/checkout@v5
     - name: Run tests