Add WGS84 <-> EGM2008 conversion

[bwinkel]

Conversion functions between WGS84 and EGM2008 reference systems should be added. While the path attenuation calculations in pathprof sub-package can use SRTM directly (as its terrain heights are relative to EGM2008 aka amsl - above mean sea level), for some other applications one would need the WGS84 heights. One example would be to derive topocentric locations of an observer.