Change pandas to never let objects share _data, create views instead

[nickeubank]

Closes Issue #11814

Several pandas operations create objects that share _data objects. In particular, all of the following result in df_news and df sharing the same _data (i.e. df_new._data is df._data returns True).

df = DataFrame({'col1':[0,1,2], 'col2':[2,3,4]})
df_new = DataFrame(df)
df_new1 = df.loc[:,:]
df_new2 = df.iloc[:,:]

Needed for PR #11500

[jreback]

you need to define BlockManager.view() this will do all of the heavy lifting, everything else will flow from there

[jreback]

xref #11970

this idea will be incorporated there.

If you want to update before then (as this might take a while), feel free to reopen.