working on FEC tests again

• pass n to try_all_decoding_combinations and don't try to decode when shards have been recoded (\#shards > n) and there are no recoded shards in the k combination under review (\max(is) < n)
• pass recoding_steps and should_not_be_decodable as arguments to end_to_end_with_recoding_template
• fix n = 5 => this leads to tests that run in less than 10sec again
• add (k, n) = (8, 10) => tests still run in less than 13sec
• split recoding scenarii into "simple" and "chain"
• show indices in a "pretty" format, i.e. showing indices greater than n as (n), (n + 1), ...