Fix writing out HeterodynedData object when data standard devaitions are supplied
Currently, writing out a HeterodynedData
object to a HDF5 file fails if the standard deviation values are user supplied rather than calculated on-the-fly. This MR fixes this by leaving the vars
attributes in the bad_slots
list - removing it wasn't required as the sigma values are added to their own dataset within the HDF5 file anyway.