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.

Makefile

@@ -11,6 +11,14 @@ NC     := \033[0m
 
 GOFLAGS=
 LDFLAGS=
+
+# The SIMD code (pkg/obikmer, simd package of Go >= 1.27) is only compiled with
+# GOEXPERIMENT=simd; without it, a portable scalar code is used instead.
+# The binaries select the vector width at run time (and fall back on an
+# emulation without SIMD hardware), so they run on any CPU.
+# Use `make GOEXPERIMENT= obitools` to build without SIMD.
+GOEXPERIMENT?=simd
+export GOEXPERIMENT
 GOCMD=go
 GOBUILD=$(GOCMD) build $(GOFLAGS) $(if $(LDFLAGS),-ldflags="$(LDFLAGS)")
 GOGENERATE=$(GOCMD) generate