Keep track of BeamPattern within Telescope, not within InterferometerSimulation

[lmachadopolettivalle]

BeamPattern is currently instantiated within InterferometerSimulation, but it is more accurately a property of any given Telescope. Per @sfiruch 's suggestion, we should instead create and keep track of BeamPattern instances within the Telescope class.

Additionally, BeamPattern seems to be only used within long observations. We should look into using it for normal observations (i.e. those generated with Observation instances instead of ObservationLong).