print nothing if sensor_sampling_rate is not invertable

The _format_headers() function in IAGA2002Writer.py would choke when
the sensor_sampling_rate parameter in the trace.stats metadata could
not be inverted. This occurred, at least, when the "Digital Sampling"
IAGA2002 header line was not parsed properly, or was blank.

geomagio/iaga2002/IAGA2002Writer.py

@@ -87,11 +87,15 @@ class IAGA2002Writer(object):
                 self._format_header("Sensor Orientation", stats.sensor_orientation)
             )
         if "sensor_sampling_rate" in stats:
-            buf.append(
-                self._format_header(
-                    "Digital Sampling", str(1 / stats.sensor_sampling_rate) + " second"
+            try:
+                buf.append(
+                    self._format_header(
+                        "Digital Sampling",
+                        str(1 / stats.sensor_sampling_rate) + " second",
+                    )
                 )
-            )
+            except TypeError:
+                buf.append(self._format_header("Digital Sampling", ""))
         if "data_interval_type" in stats:
             buf.append(
                 self._format_header("Data Interval Type", stats.data_interval_type)