Add back-to-back-stem-and-leaf plots

[tobyWorland]

Fulfills issue #1

[CLAassistant]

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[Symbolics]

Looks good. Two things:

1. Will you please make split-stems a keyword option in stem-and-leaf. This way the usage would be something like: (stem-and-leaf x :split-stems t). The keyword defaults should be nil
2. Squash all commits with a single commit message like "implement back to back stem plots". See how to write a good commit message for some guidelines.

[Symbolics]

I've just edited my comment. I had split brain syndrome :-) because initially we were discussing split-stems and this PR implements back-to-back. We really shouldn't mix &optional and &keys parameters, so back-to-back does need to be separate since it takes both x and y. split-stems we can signal with a keyword to stem-and-leaf.

Thanks for the contribution!

[tobyWorland]

Thanks for the review!

initially we were discussing split-stems and this PR implements back-to-back.

Ah, I think you're confusing me for @jg20019. I have not participated in the discussion for the other issue.

Squash all commits with a single commit message like "implement back to back stem plots".

Done

Will you please make split-stems a keyword option ...

Done, plus I added it to the docstring.

[Symbolics]

Well done, thanks!