Change the way sequence definition are managed. They are now when present stored as an attribute

Former-commit-id: 6e618377c05b42937d2eace3c9668390980ab68c

pkg/obiformats/empty_file.go

@@ -0,0 +1,9 @@
+package obiformats
+
+import "git.metabarcoding.org/lecasofts/go/obitools/pkg/obiiter"
+
+func ReadEmptyFile(options ...WithOption) (obiiter.IBioSequence, error) {
+	out := obiiter.MakeIBioSequence()
+	out.Close()
+	return out, nil
+}