wifi: WifiMode refactor

This is a WIP on WifiMode refactor. Current tasks:

• Fix WifiMode::IsHigherDataRate
• Split the commit into 3 parts:
  • refactor PHY entities to prepare for WifiMode
  • refactor WifiMode: Implement callback
  • refactor WifiMode: other refactor that is not related to callback (e.g., IsHigherCodeRate and IsHigherDataRate)

IsHigherDataRate problem

I tried to change it to only compare constellation size as per @sderonne suggestion, but it does not work. So I tried to make a "truth table" to investigate the problem.

	DSSS	HR-DSSS	ERP-OFDM	OFDM	HT	VHT	HE
DSSS	const	FALSE	FALSE	FALSE	FALSE	FALSE	FALSE
HR-DSSS	TRUE	const	const	const	const	const	const
ERP-OFDM	TRUE	!const	const & code	const & code	const & code	const & code	const & code
OFDM	TRUE	!const	const & code	const & code	const & code	const & code	const & code
HT	TRUE	!const	const & code	const & code	const & code	const & code	const & code
VHT	TRUE	!const	const & code	const & code	const & code	const & code	const & code
HE	TRUE	!const	const & code	const & code	const & code	const & code	const & code

• The first column is the modulation class of current mode, and the first row is the modulation class to be compared to.
• "const" means it will compare constellation size (i.e., return (GetConstellationSize () > other.GetConstellationSize ());)
• "!const" means the check is flipped (return (other.GetConstellationSize () > GetConstellationSize ());).
• "const & code" means it will compare constellation size, and additionally the code rate if both has the same constellation size (i.e., if (GetConstellationSize () == other.GetConstellationSize ()) { return IsHigherCodeRate (other); }).

As we can see, there are some observation:

• if the current mode is DSSS it always return FALSE, while if the other mode is DSSS it always return TRUE
  • the exception is when both uses DSSS, then compare constellation size
• if current mode is HR/DSSS then compare constellation size
• if current mode is > HR/DSSS while other is HR/DSSS, then compare constellation size but flipped (return other > current)
• if either is at least using ERP-OFDM, then compare constellation size, if both is equal, compare code rate

My reimplementation of this is in commit 289a51d6