Check if the data set is available on the local machine already.

def data_available(dataset_name=None):
    """Check if the data set is available on the local machine already."""
    try:
        from itertools import zip_longest
    except ImportError:
        from itertools import izip_longest as zip_longest
    dr = data_resources[dataset_name]
    zip_urls = (dr['files'], )
    if 'save_names' in dr: zip_urls += (dr['save_names'], )
    else: zip_urls += ([],)

    for file_list, save_list in zip_longest(*zip_urls, fillvalue=[]):
        for f, s in zip_longest(file_list, save_list, fillvalue=None):
            if s is not None: f=s # If there is a save_name given, use that one
            if not os.path.exists(os.path.join(data_path, dataset_name, f)):
                return False
    return True