lr-wpan: Adds PHY rx Sensitivity configuration support

Most IEEE 802.15.4 radios offer the possibility of changing the RX sensitivity as well as the transmit power. In its current state, the PHY can change its transmit power but it does not offer the possibility for a quick change of the RX sensitivity parameters. This MR adds the simple get-set functions to do that. Likewise, I added the possibility to change the ED threshold, which seems to be possible in some radio transceivers. This is in response to the original request/suggestion by @TorbenPetersen.

For sensitivity, all radios follow the standard and only accept a minimum of -85dBm. However, most vendors offer higher sensitivity, but for the most part, high sensitivity is considered to be -95 ~ -96dBm for IEEE 802.15.4. Ns-3 is set with a default ultra-high sensibility of -106.58dBm (The reason for this is unknown, perhaps this is the default for some Wifi radios?) I left the value untouched, but I think it would recommend adjusting it to be closer to the value of many vendors.

Likewise, ns-3 transmit power default value seems to be 0dBm which is within the normal range (-3dBm-10dBm) but something like 3 dBm seems more appropriate.

Edit: Status, This MR is complete and ready to be pushed.