v1.3.0

Make the default noise background generation follow chi-squared distribution, appropriate for radiometer noise. Noise is calculated using data frame resolutions, and scaled up using a mean value. This change affects frame.add_noise() and frame.add_noise_from_obs(). Using a normal distribution instead for noise is still possible, using the noise_type argument, but for most (high spectral resolution) cases, using chi-squared noise is the best option.