ipa generate_input --test_case_index

When running `python ipa generate_input` it needs a test case index.  This isn't very well explained, but I found that a value of 0 worked.

Please document this flag better in the README.  It isn't easy to work out what to do when running the instructions.  A default value might be a good idea so that people can get started without having to engage with this option.
