Bug 2400

This is to track the review of SpectrumWifiPhy, which is proposed to allow Wi-Fi devices to connect to Spectrum channels.  This will allow SpectrumWifiPhy to receive signals that are not Wi-Fi, such as in Wi-Fi/LTE coexistence, or interference scenarios.

This version of SpectrumWifiPhy is very basic, in that it is for the most part a clone of YansWifiPhy, with some adaptor code at the very bottom of the Phy where the channel interconnection occurs.  Instead of YansWifiChannel, a SpectrumChannel type must be used (see src/spectrum).

The main difference is that when a foreign (non-Wi-Fi) signal is received, it is simply added to the InterferenceHelper and evaluated for CCA BUSY status, without further receive processing.  The performance of SpectrumWifiPhy and YansWifiPhy should be identical.

Three example programs are provided, and one test suite.  The examples allow users to substitute SpectrumWifiPhy and YansWifiPhy under different conditions (signal loss, interference, saturating traffic) and compare results.

In the long run, SpectrumWifiPhy will use different interference trackers and error models more suited for the spectrum framework, but for the next release, it is proposed to simply reuse the existing Yans-compatible model and interference helper.

This version of SpectrumWifiPhy is known to lack two final things necessary for merging to ns-3-dev, IMO:
1) update the Wi-Fi .rst documentation accordingly
2) it is presently hardwired to the lower U-NII bands (5.180 GHz) and needs to be generalized (mainly, this involves making the spectrum model code more configurable)

However, I wanted to get the code review started while the above two are finished off.

This is a large patch and users can experiment with it by cloning the spectrum-wifi branch from github: https://github.com/tomhenderson/ns-3-dev-git/commit/7b6d894fcd8f82fb017654c9c6a95f58cabcfe02

However, I will also post a Rietveld issue to collect comments on the code.