Start the implementation of the TX API

New features

• The TX API is following a similar approach with the RX, using encoder classes that extend the base encoder() class. The encoders are using PMT messages for the input and output data. Output PMTs should be compatible with the PDU to tagged stream. Combined with Tagged Stream Multiply Length Tag and Burst Shaper blocks, we can support bursty transmissions relatively easy.
• Implement a parameterizable IEEE 802.15.4 frame encoder

Fixes

• Fix #169 (closed)
• Fix #174 (closed). The solution to this was to integrate a slightly modified Constellation Modulator. Now the tags for both FSK and PSK modulations are placed exactly at the beginning of the frame.

The idea is to use the tag to properly instruct the device for a bursty transmission. This will be done through gr-soapy. For more details refer to librespacefoundation/gr-soapy#49