avoid demodulator concurrency

* this frees up resources used by the current demodulator before
  starting a new one
* this addresses an issue where users of single-channel AMBE sticks
  could not seamlessly switch between digital modes

csdr/chain/dummy.py

@@ -0,0 +1,14 @@
+from pycsdr.types import Format
+from csdr.chain import Module
+
+
+class DummyDemodulator(Module):
+    def __init__(self, outputFormat: Format):
+        self.outputFormat = outputFormat
+        super().__init__()
+
+    def getInputFormat(self) -> Format:
+        return Format.COMPLEX_FLOAT
+
+    def getOutputFormat(self) -> Format:
+        return self.outputFormat