BUG: Fix parsing message 13 in NEXRAD

Commit f021338 refactored the offset handling, but left an
off-by-one error. This produced some warnings, but they were not
caught--so now add to the tests to check the amount of warnings that get
spit out.

src/metpy/io/nexrad.py

@@ -459,8 +459,8 @@ def _decode_msg13(self, msg_hdr):
             num_az = 360
             chunk_size = 32
             bit_conv = Bits(16)
+            offset = 0
             for e in range(num_el):
-                offset = e * num_az * chunk_size
                 seg_num = data[offset]
                 if seg_num != (e + 1):
                     log.warning('Message 13 segments out of sync -- read {} but on {}'.format(
@@ -473,6 +473,7 @@ def _decode_msg13(self, msg_hdr):
                         gates.extend(bit_conv(data[offset + i]))
                     az_data.append(gates)
                 self.clutter_filter_bypass_map['data'].append(az_data)
+                offset += num_az * chunk_size + 1
 
             if offset != len(data):
                 log.warning('Message 13 left data -- Used: %d Avail: %d', offset, len(data))

tests/io/test_nexrad.py

@@ -23,28 +23,32 @@
 
 # 1999 file tests old message 1
 # KFTG tests bzip compression and newer format for a part of message 31
-# KTLX 2015 has missing segments for message 18, which was causing exception
-level2_files = [('KTLX20130520_201643_V06.gz', datetime(2013, 5, 20, 20, 16, 46), 17, 4, 6),
-                ('KTLX19990503_235621.gz', datetime(1999, 5, 3, 23, 56, 21), 16, 1, 3),
+# KTLX 20150530 has missing segments for message 18, which was causing exception
+# KICX has message type 29 (MDM)
+level2_files = [('KTLX20130520_201643_V06.gz', datetime(2013, 5, 20, 20, 16, 46), 17, 4, 6, 0),
+                ('KTLX19990503_235621.gz', datetime(1999, 5, 3, 23, 56, 21), 16, 1, 3, 0),
                 ('Level2_KFTG_20150430_1419.ar2v', datetime(2015, 4, 30, 14, 19, 11),
-                 12, 4, 6),
-                ('KTLX20150530_000802_V06.bz2', datetime(2015, 5, 30, 0, 8, 3), 14, 4, 6),
-                ('KICX_20170712_1458', datetime(2017, 7, 12, 14, 58, 5), 14, 4, 6),
-                ('TDAL20191021021543V08.raw.gz', datetime(2019, 10, 21, 2, 15, 43), 10, 1, 3),
+                 12, 4, 6, 0),
+                ('KTLX20150530_000802_V06.bz2', datetime(2015, 5, 30, 0, 8, 3), 14, 4, 6, 2),
+                ('KICX_20170712_1458', datetime(2017, 7, 12, 14, 58, 5), 14, 4, 6, 1),
+                ('TDAL20191021021543V08.raw.gz', datetime(2019, 10, 21, 2, 15, 43), 10, 1,
+                 3, 0),
                 ('Level2_FOP1_20191223_003655.ar2v', datetime(2019, 12, 23, 0, 36, 55, 649000),
-                 16, 5, 7)]
+                 16, 5, 7, 0)]
 
 
 # ids here fixes how things are presented in pycharm
-@pytest.mark.parametrize('fname, voltime, num_sweeps, mom_first, mom_last', level2_files,
-                         ids=[i[0].replace('.', '_') for i in level2_files])
-def test_level2(fname, voltime, num_sweeps, mom_first, mom_last):
+@pytest.mark.parametrize('fname, voltime, num_sweeps, mom_first, mom_last, expected_logs',
+                         level2_files, ids=[i[0].replace('.', '_') for i in level2_files])
+def test_level2(fname, voltime, num_sweeps, mom_first, mom_last, expected_logs, caplog):
     """Test reading NEXRAD level 2 files from the filename."""
+    caplog.set_level(logging.WARNING, 'metpy.io.nexrad')
     f = Level2File(get_test_data(fname, as_file_obj=False))
     assert f.dt == voltime
     assert len(f.sweeps) == num_sweeps
     assert len(f.sweeps[0][0][-1]) == mom_first
     assert len(f.sweeps[-1][0][-1]) == mom_last
+    assert len(caplog.records) == expected_logs
 
 
 @pytest.mark.parametrize('filename', ['Level2_KFTG_20150430_1419.ar2v',