CSFC: Calculated / Custom Surface-Level Modules

    All variables found within the CSFC module are "custom" or "calculated" variables (i.e.
    these variables are not found/selectable from ECMWF reanalysis and must be calculated
    from other variables).  These variables defined only over the "surface", which is along
    the surface orography.

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

    Examples of CSFC variables are:
        - Tropopause Height
        - Eddy Kinetic Energy

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