Add detected features to L2 product

[zxdawn]

The features can be useful to find the plume source location:

HyperGas/hypergas/hyper.py

Lines 426 to 445 in 7ea98bf

	def plume_mask(self, varname='ch4_comb_denoise', n_min_threshold=5, sigma_threshold=1):
	"""Create a priori plume masks
	Args:
	scn (Satpy Scene):
	Scene including one variable named "segmentation" which is calculated by `Land_mask`
	segmentation (xarray DataArray):
	0: ocean, >0: land
	varname (str):
	The variable used to create plume mask. (Recommend: <gas>_comb_denoise)
	n_min_threshold (int):
	The minimum number of points per threshold for detecting features.
	Default: 5
	sigma_threshold (int):
	Gaussian filter sigma for smoothing field.
	Default: 1. Because the <gas>_comb_denoise field is already smoothed, 1 should be high enough.
	"""
	thresholds, features, da_plume_mask = Mask(self.scene, varname, n_min_threshold=n_min_threshold, sigma_threshold=sigma_threshold).get_feature_mask()
	return da_plume_mask

We can add it to L2 NC files.