Returns the name of the variable that defines the Z axis or height/depth

    :param netCDF4.Dataset nc: netCDF dataset

def get_z_variable(nc):
    '''
    Returns the name of the variable that defines the Z axis or height/depth

    :param netCDF4.Dataset nc: netCDF dataset
    '''
    axis_z = nc.get_variables_by_attributes(axis='Z')
    if axis_z:
        return axis_z[0].name
    valid_standard_names = ('depth', 'height', 'altitude')
    z = nc.get_variables_by_attributes(standard_name=lambda x: x in valid_standard_names)
    if z:
        return z[0].name
    return