Rename `s2_secondary_sc_gain_mc` to `se_gain_mc`

[dachengx]

fuse/fuse/plugins/detector_physics/electron_extraction.py

Lines 28 to 35 in ad1789c

	s2_secondary_sc_gain_mc = straxen.URLConfig(
	default="take://resource://"
	"SIMULATION_CONFIG_FILE.json?&fmt=json"
	"&take=s2_secondary_sc_gain",
	type=(int, float),
	cache=True,
	help="Secondary scintillation gain [PE/e-]",
	)

Similar to https://github.com/XENONnT/cutax/issues/515, the s2_secondary_sc_gain_mc is essentially SEG. To be consistent and help understanding, s2_secondary_sc_gain_mc should be renamed to se_gain_mc.