For now the generic clear text message sink just prints the decoded
clear text messages in the stdout. In the future and depending the needs
of the project, the clear text messages can be forwarded at other
services on the same host or via the network.

Also the debug message source block now supports injection of artificial
errors, for more robust decoding of the Morse decoder block.

The Morse decoder seems that works properly now. However, there is no
recovery strategy yet from undecodeable symbol sequences. 

Now, with the Morse code debug source block the testing of the decoder
can begin.

Add at the examples directory a small flowgraph that connects the debug
source block with the decoder.

Depending the received symbol the tree is traversed towards the leafs.
At each step the corresponding decode-able character is appended at a
string buffer. 

TODOs: 
* Add logic in the decoder to retrieve the decoded words from the tree
* Create a debug source block  

The morse decoder is not yet functional. The decoding is performed
using a binary tree. Left child, corresponds to a received dot whereas
right to a received dash.

* The matched filter makes use of VOLK
* The examples directory contains a simple flowgraph that demonstrates
the performance of the matched filter