DRAFT: lr-wpan: Adds PHY preamble and RSSI support

This MR adds support for preamble and RSSI for ns-3 Lr-Wpan.

Preamble design for ns-3 lr-wpan

• With the introduced changes, the preamble is mainly used to register the initial signal into the spectrum model to initiate tracking of the signal during the packet transmission.
• Additional events for the end of the SFD or the PHR were not created as their introduction seemed unnecessary.
• The RSSI probing currently takes place after the preamble. According to the standard, it should be done in the PHR, but there did not seem to be any significant difference for the ns-3 implementation, and doing it in the PHR would require introducing extra events solely for this purpose (breaking the packet into an additional chain of events).
• No additional PHY states were added. The preamble takes place during RX_ON, and the rest of the packet takes place during the BUSY_RX state. This was a conscious design choice of not adding an additional "PREAMBLE" state, to mirror the PHY state diagram in which the PHY is based,and that only uses 2 states to receive the packet (atmel'S at86rf233). This potentially means that we could be receiving 2 preambles from different packets at the same time. This is ok, because the remaining part of the 2nd packet would not be processed because the packet would not be in the right state.

Please take a look at the code and, based on your experience, give me your opinion on these design choices or other considerations that I should take. I am also looking for ideas for adding Preamble or RSSI new tests.