bugfix in Sentinel1BurstSlc.swath_name() (#89)

Author: yunjunz

src/s1reader/s1_burst_slc.py

@@ -668,7 +668,7 @@ def width(self):
     @property
     def swath_name(self):
         '''Swath name in iw1, iw2, iw3.'''
-        return self.burst_id.swath_name
+        return self.burst_id.subswath.lower()
 
     @property
     def thermal_noise_lut(self):