Inform the user whether multiprocessing was used

[tadamcz]

Hi, thanks for this cool library.

One thing that would be nice would be to tell the user what swifter decided to do, e.g.:

• was it able to vectorize?
• did it choose to apply multiprocessing with Dask?

Right now it seems everything is totally transparent to the user; I cannot easily tell if swifter is even using more than one core.

[jmcarpenter2]

This makes sense.. how do you envision sharing this information with the user? Logging it? Displaying it in the apply progress bar?