Refactoring of get_sun_buffer
Created by: tobiaslcmueller
Pulled out Asymptotic mapping from channel-specific get_buffer functions to reduce code-doubling -> Noe measurable performance effect
Additionally refactored mapping to use explicit nested if-else instead of heavyside functions -> test with 2l, Kagome, 150/30/20 frequencies down to Lambda=2 prdouced speedup of ca. 5%