Bandpasses for LSST need an update

[rbiswas4]

LSST bandpasses provided by the Rubin Observatory have been updated on 5/7/21, while the sncosmo builtins have been last updated in 2016. Such updates are always in the throughputs repository: https://github.com/lsst/throughputs

The complete updates are found in https://github.com/lsst/throughputs/tree/main/baseline (version 1.7 seems the current one)

In sncosmo, we usually use truncated bandpasses which remove the bandpass values that are below a certain threshold 10**(-250), in order to save time in magnitude calculations. This is done by the script https://github.com/lsst/throughputs/blob/main/approximate_baseline/approximateBandpasses.py
and stored in the approximate_baseline directory of the repository e.g : https://github.com/lsst/throughputs/blob/main/approximate_baseline/total_g.dat

The total indicates that it uses the atmosphere as 1.2 X throughout.

• In previous versions, this made a lot of sense, as the threshold only removed values that were recorded in the csv files as zero. We should manually check if these approximate throughputs still make sense (does not lead to patchy bandpasses)
• The sncosmo builtins for lsst bandpasses should be updated to these throughputs.