wifi: Add Wifi Channel Occupancy Helper (WifiCoHelper)

Add a new helper class called WifiCoHelper in wifi module. Channel occupancy is fraction of time a Phy spends in one of the following PhyStates: IDLE, RX, TX, CCA_BUSY. The helper class can measure channel occupancy on all Phys associated with a WifiNetDevice.

• Add constructor that takes start and stop time arguments
• Rename file (and class) to WifiCoTraceHelper
• Align field widths in statistics stream output, such as:

---- COT for 0:0 ----
Showing duration by states:
IDLE:       +98.745ms   (2.47%)
CCA_BUSY:   +36.192ms   (0.90%)
TX:        +106.612ms   (2.67%)
RX:          +3.75845s (93.96%) 

• Rename the test from wificohelper to wifi-co-trace-helper
• Add illustrative example program, perhaps showing that STAs in same AC achieve the same airtime fairness in the long run, while a BSS with STAs using different ACs will have different results. This example could be also reused across the trace helpers.
• (to be determined) Extend the PrintStatistics() method to use object names (e.g., AP, wifi0) if available.
• Add any missing Doxygen