Add test to write to local use.

tests/test_grib2_write_local_use.py

@@ -0,0 +1,27 @@
+import grib2io
+import numpy as np
+import pytest
+
+def test_grib2_write_local_use(tmp_path, request):
+    target_dir = tmp_path / "test_grib2_write_local_use"
+    target_dir.mkdir()
+    target_file = target_dir / "testwrite_localuse.grib2"
+
+    grib2file = request.config.rootdir / 'tests' / 'input_data' / 'gfs_20221107' / 'gfs.t00z.pgrb2.1p00.f012_subset'
+    grib2out = grib2io.open(target_file, mode='w')
+    with grib2io.open(grib2file) as g:
+        msg = g[0]
+        print(type(msg))
+        newmsg = grib2io.Grib2Message(msg.section0, msg.section1, b"HELLO WORLD!",
+                                      msg.section3, msg.section4,
+                                      msg.section5)
+        newmsg.data = np.copy(msg.data)
+        newmsg.pack()
+        grib2out.write(newmsg)
+
+    grib2out.close
+
+    g = grib2io.open(target_file)
+    msg = g[0]
+    assert msg.section2 == b"HELLO WORLD!" 
+    g.close()