add tests

tests/test_surv.py

@@ -0,0 +1,22 @@
+from pyModeS import surv
+
+
+def test_fs():
+    assert surv.fs("2A00516D492B80")[0] == 2
+
+
+def test_dr():
+    assert surv.dr("2A00516D492B80")[0] == 0
+
+
+def test_um():
+    assert surv.um("200CBE4ED80137")[0] == 9
+    assert surv.um("200CBE4ED80137")[1] == 1
+
+
+def test_identity():
+    assert surv.identity("2A00516D492B80") == "0356"
+
+
+def test_altitude():
+    assert surv.altitude("20001718029FCD") == 36000