bessel and truncated normal beam shapes

[adokter]

Currently the beam shape function in integrate_to_ppi() is normal. Verify the effect of truncating the normal distribution outside the 3dB points, and/or using a bessel function to approximate the power distribution.

[adokter]

Beamshapes for first order Bessel function is very similar to a Gaussian, therefore holding off on implementation.

[adokter]

Added truncated normal distribution for testing only, available in beam_profile branch: 6f87cb8