API: deprecate DataFrame.interpolate

add a more useful warning message

use the correct tense

DOC: rls notes
Merge pull request #3675 from cpcloud/remove-interpolate-3582

API: deprecate DataFrame.interpolate
BUG: inserting dups into blocks ok

CLN: finished duplicate item insert/delete - whoosh! (GH3679)

BUG: insert almost working

BUG: fixed insertion of dup columns!

ENH: added allow_duplicates kw to DataFrame.insert to indicate that inserting of

     a duplicate column should be allowed (default is False)