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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_heat_kernel(network_id):
 """Return the identifier of a heat kernel calculated for a given network. Parameters network_id : str The UUID of the network in NDEx. Returns ------- kernel_id : str The identifier of the heat kernel calculated for the given network. """

    url = ndex_relevance + '/%s/generate_ndex_heat_kernel' % network_id
    res = ndex_client.send_request(url, {}, is_json=True, use_get=True)
    if res is None:
        logger.error('Could not get heat kernel for network %s.' % network_id)
        return None

    kernel_id = res.get('kernel_id')
    if kernel_id is None:
        logger.error('Could not get heat kernel for network %s.' % network_id)
        return None
    return kernel_id