fsk_demodulator: invert symbols in non-IQ if deviation is negative

We use a negative deviation to indicate that the high tone encodes
the bit 0 and the low tone encodes the bit 1. With IQ input, this
is taken into account for quadrature demodulation. However, with
non-IQ input, which is already FM-demodulated, this wasn't taken
into account. In this case it is necessary to invert the stream
of symbols at some point.

See the discussion in
#307 (comment)

python/components/demodulators/fsk_demodulator.py

@@ -168,7 +168,13 @@ def __init__(self, baudrate, samp_rate, iq, deviation=None,
             self.connect(self.dcblock, self.clock_recovery)
             if dump_path is not None:
                 self.connect(self.dcblock, self.waveform)
-        self.connect(self.clock_recovery, self)
+        if not iq and _deviation < 0:
+            # when working with FM-demodulated input, if the deviation is
+            # negative, the polarity of the signal needs to be inverted.
+            self.invert_polarity = blocks.multiply_const_ff(-1, 1)
+            self.connect(self.clock_recovery, self.invert_polarity, self)
+        else:
+            self.connect(self.clock_recovery, self)
 
     _default_clk_rel_bw = 0.06
     _default_clk_limit = 0.004