Handle topography shadow

[mjtueros]

• Check if xmax (or even beter a region around it) is visible by the antenna, and by the core position.
• Make a table of altitude above terrain For antennas and core
• Make a function to evaluate fresnel zones and decide if we keep the antenna/shower or not. (or flag it)
• As you are at it, compute the antenna viewing angle (and its range) so that the antenna simulation is more accurately piking the direction from the antenna response file.
  -Save distance from xmax to antenna for future use? (in sim2root)
  -Save w angle to antenna for future use?