Improve the calculation of the spot beam coverage area

[edgar-souza]

When the satellite elevation angle is 90 deg, the coverage area of a circular beam is given by the curved surface area of the spherical cap. This is not the case for other elevation angles and this calculation has to be improved.

[edgar-souza]

There is an implementation of the method for generating footprints (Siocos,1973) which is found in the book "Satellite Communication Systems" by M. Richharia. For the moment, the implementation is available in branch 'dev-area': https://github.com/SIMULATOR-WG/SHARC/blob/dev-area/sharc/support/footprint.py