[Feature] Better NO2 segmentation

[zxdawn]

S5P-LNO2/main/s5p_lnox_utils.py

Lines 237 to 251 in 0cd6465

	def segmentation(scn, min_threshold=4e14, max_threshold=1e15, step_threshold=2e14):
	'''Segmentation the NO2 field by tobac
	Tobac: https://gmd.copernicus.org/articles/12/4551/2019/
	'''
	logging.info(' '*4 + 'Segmentation of NO2 ...')
	# get the finite no2 data
	no2_finite = get_finite_scn(scn) # unit: molec./cm2
	# Detect the features and get the masks using tobac
	masks_scn = feature_mask(no2_finite, min_threshold=min_threshold,
	max_threshold=max_threshold, step_threshold=step_threshold)
	return masks_scn

The threshold is hardcoded. A better method is to use the monthly average NO2 as the minimum value of the threshold.

Because we are developing S5P-LNO2 for Arctic lightning NO2 (background NO2 is quite low), this doesn't matter at the current stage.

Please feel free to PR for making it work for polluted regions!