Fix okumura-hata propagation loss model according to 4.4.2 in the COST 231 final report

okumura-hata propagation loss model according to equation (4.4.2) in the COST 231 final report is:

a(hMobile) = (1.1*log f − 0.7) * hMobile/m − (1.56 ⋅log f − 0.8)