ENH : First attempt to add sparse_pivot

[yupbank]

• Resolves ENH: Add sparse option in pivot_table / pivot / unstack #14493
• tests added / passed
• passes git diff upstream/master | flake8 --diff
• whatsnew entry

sparse_series = pivot_sparse(index=df['a'], columns=df['b'], values=df['c'])

[jreback]

this would need to be an actual sparse pivot implementation. converting to dense then pivotting doesn't really work here. If you have some thoughts on that, pls reopen / comment.

[yupbank]

hmm.. i didn't convert to dense though...

the idea is just transforming the index instead of data/values. it's like keep the data in a series and update the new index which is after pivot