DSFC: Dry Surface-Level Modules

    All variables found within the DSFC module are "dry" variables (i.e. not explicitly
    affected by water and/or convection), and are considered to be "surface-level"
    variables by ECMWF reanalysis. This means that they are variables defined only over the
    "surface", which could be either at sea-level-pressure or along the surface orography.

    To select this module in erainitialize, use:
        - erainitialize(modID="dsfc")

    Examples of DSFC variables are:
        - Surface wind (all three directions)
        - Surface pressure (along orography)
        - Sea-level pressure (at sea-level height)

    In ERA5, DSFC variables are identified as:
        - reanalysis-era5-single-levels

    In ERA-Interim, DSFC variables are identified as:
        - levtype: sfc

    Note: If you are interested in collaborating to add other variables this feature,
    please contact me (Nathanael), and/or create a pull request.
