Output sequence length information for repertoire comparison

[BrandenOlson]

It would be helpful for the user to see statistics on the differences in sequence length distribution when comparing two repertoires at a time, in particular when using the sequence_alignment and/or germline_alignment columns. Thus, any large differences in these lengths can be easily observed by the user. Perhaps we can specifically output a warning when the absolute difference in median sequence length, e.g., is higher than some threshold? Deciding this threshold will be subtle, so I'm happy to hear any thoughts on this.