CLN depreciate save&load in favour of to_pickle&read_pickle

DOC replace save&load with to_pickle&read_pickle
FIX not mention save/load being removed
Merge pull request #3787 from hayd/read_pickle_and_to_pickle

CLN deprecate save&load in favour of to_pickle&read_pickle
FIX/ENH: attempt soft conversion of object series before raising a TypeError when plotting