Implement back-to-back stemplot

[Symbolics]

The code for a single stemplot should point the way. I would start by modifying the code to write the leaves for each stem into a single string (one string for each set of leaves, i.e. two leaf strings per stem, one for each side), and then determining the length of the largest on the left-hand side. This will give you the padding (mincol) to use in format. Once the padding is known, loop through the stems and:

1. Print left side leaf string, right padded to the largest string
2. Print the stem
3. Print the right-side leaf string

[Symbolics]

Closed in commit 9a62112