Flatten in case column contains string and array

[sprmnt21]

as described in this discussion, the result of flattening a dataframe on a column containing arrays and strings is, in my opinion, unexpected.
A possible modification could include at least one keyword that optionally allows the possibility of managing the scalar-string elements as vectors of length 1.

[bkamins]

I have opened #2767 in parallel with a more elaborate discussion of the issue. I will add a proposal of kwarg there.