def dict_add(dict_use, key, obj):
    """Simple wrapper to add obj to dictionary if it doesn't exist. Used in fitter.Fitter when defining defaults

    Parameters
    ----------
    dict_use: Dictionary
        dictionary to be, possibly, updated
    key: str
        key to update, only updated if the key doesn't exist in dict_use already
    obj: Object
        Object to be added to dict_use under key

    Returns
    -------
    dict_add: Dictionary
        updated dictionary
"""
    dict_res = dict_use.copy()
    if key not in dict_res:
        dict_res[key] = obj
    return dict_res