GeometryBeamBlockFrac
returns the partial beam blockage fraction
From Bech et al. (2003), Eqn 2 and Appendix
GeometryBeamBlockFrac(Th, Bh, a)
Bech et al, 2003 The Sensitivity of Single Polarization Weather Radar Beam Blockage Correction to Variability in the Vertical Refractivity Gradient American Meteorological Society, AMS journals Volume 20 Issue 6