CPRE: Calculated / Custom Surface-Level Modules

    All variables found within the CPRE 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 are saved at a predefined list of 37 pressure
    levels from 1000hPa to 1hPa.

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

    Examples of CPRE variables are:
        - Meridional Streamfunction
        - Eddy Heat Fluxes
        - Moisture Fluxes

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