ENH/TST: testing of panel constructors (GH #797)
         added convert_dates='coerce' option to convert_objects, to force
           conversions to datetime64[ns]
ENH/BUG: where ops now have a full test suite
         where can upcast integer to float as needed (on inplace ops #2793)

DOC: added upcasting gotchas docs
     hopefully fixed py3 isses with internals.Block.where raising