update default parameters + python updates

- by default ggwave will now output 32-bit float samples
- python samples no longer use numpy
- python test now decodes payload
- fix buffer overflow in receive.py example

tests/test-ggwave.py

@@ -1,6 +1,15 @@
+import sys
 import ggwave
 
 instance = ggwave.init()
 
+payload = 'hello python'
+
 # generate audio waveform for string "hello python"
-waveform = ggwave.encode("hello python", txProtocolId = 1, volume = 20, instance = instance)
+waveform = ggwave.encode(payload, txProtocolId = 1, volume = 20, instance = instance)
+
+# decode the audio waveform back to text
+res = ggwave.decode(instance, waveform)
+
+if res != payload.encode():
+    sys.exit(1)