function for iceflux at 260K
Finney proposed iceflux at 440 hPa as proxy for lightning. Equation 9 in: https://doi.org/10.5194/acp-14-12665-2014
Romps notes that it would be better to compute the iceflux on -13C = 260K isotherm: https://doi.org/10.1029/2019GL085748
Thus, it would be good to have a function to compute this iceflux for a column of ERA5 profiles
Function should go here: https://git.uibk.ac.at/c4031042/ewl/blob/master/R/derive_functions.R