def _unpack_parameters(parameters, key):
    """Unpacks the values from a dictionary containing various parameters for
    pymaws

    Parameters
    ----------
    parameters : dict
        planetary parameters dict with keys:
            angular_frequency: float, (rad/sec)
            gravitational_acceleration: float, (m/sec^2)
            mean_radius: float, (m)
            layer_mean_depth: float, (m)
    key : String
        key with the one of the names of the above keys.
    Returns
    -------
    value : Float, scalar
            one of the above values
    """
    if not isinstance(key, str):
        raise TypeError(str(key) + ' should be string...')
    if not isinstance(parameters, dict):
        raise TypeError('parameters should be dictionary...')
    if key not in parameters:
        raise KeyError(str(key) + ' not in parameters!')
    value = parameters[key]
    return value
