Handle sequences with different length in logoplots

[grst]

However, when there are two different lengths of junctions of each cell, it will return an error when I plot the CDR3-motif with following code.

There's an example here how you can filter by length:
https://scirpy.scverse.org/en/latest/tutorials/tutorial_5k_bcr.html#logoplots

But this is good feedback, I think we should make it easier to make logoplots of sequences with different lengths, e.g. by automatically generating separate plots for all lengths in the input dataset.

Originally posted by @grst in #644

[grst]

Another way of addressing this is by plotting a fixed window from the start and end of the sequences only.
HT @RaphaelDeGottardi