calcAtmosCorr(): DOS4 not implemented

[fdetsch]

path <- system.file("extdata", package = "satellite")
files <- list.files(path, pattern = glob2rx("LC08*.TIF"), full.names = TRUE)
sat <- satellite(files)
sat_atmos <- calcAtmosCorr(sat, model = "DOS4", esun_method = "RadRef")

returns empty layers due to lns. 213-215 in satellite/calcAtmosCorr.R. Seems like DOS4 as suggested by Moran et al. (1992) has never been implemented, which makes the documentation in ?calcAtmosCorr rather misleading.

@tnauss, do you guys have free capacities to implement this?