New Repo: ns3-if77-module

The ns3-if77-module provides an implementation of the ns3::PropagationLossModel, which uses the If77 propagation model developed by G. D. Gierhart and M. E. Johnson. The If77 propagation model was developed in the 1970s to estimate the service coverage for radio systems. It can be used to calculate propagation loss for ground/air, air/air, ground/satellite, and air/satellite systems for frequencies in the 0.1Mhz to 20Ghz range.