Get time steps for a given MISSource:
Get the time steps available in a given dataset for a MIS.
string giving the mis; it must use the same spelling as used in mis_boundaries
string defining dataset to be downloaded (a list of possible values can be obtained with
list_available_datasets()). If set to "custom", then a single nc file is used from "path_to_nc"
the path to the custom nc file containing the palaeoclimate reconstructions. All the variables of interest need to be included in this file.