Removed white space from join string for rx IDs. Not sure why it was there.

gprMax/input_cmd_funcs.py

@@ -658,7 +658,7 @@ def rx(x, y, z, identifier=None, to_save=None, polarisation=None, dxdy=None, rot
     c = Coordinate(x, y, z)
     to_save_str = ''
     if to_save is not None:
-        to_save_str = ' '.join(to_save)
+        to_save_str = ''.join(to_save)
 
     command('rx', str(c), identifier, to_save_str)