ENH: Allow passing multiple axes as df.boxplot's `ax` argument

[onesandzeroes]

As discussed in #6991. At the moment with ax accepting a single axis you cannot map a multi-axis boxplot onto existing axes, but we could allow ax to be an array of axes. Would require:

• Checking that the number of axes passed is equal to the number of axes the boxplot
  wants to produce.
• Some documentation to make clear whether the axes get used in row or column order
  (or maybe it could support two-dimensional arrays of axes?)

[jreback]

is this a new option? #6991 closed as the related PR was merged

@TomAugspurger ?

[TomAugspurger]

iirc that PR handled it for the no subplots case (just a single axis). This would handle multiple axes.