ENH: enable mul, div on Index by dispatching to Series

[jbrockmendel]

One of the last remaining inconsistencies between Index and Series arithmetic is that division and multiplication is entirely disabled for object-dtype Index, while for Series we operate pointwise. This PR makes Index behave the same way, and simplifies the code by using existing Series-op-wrapping code to do so.

• closes #xxxx
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

I'm pretty sure there is at least one issue for this, will track it down.

[jorisvandenbossche]

Can you explain your changes?

[jreback]

are we actually testing these? it seems we are enabling all numeric methods; do all of these even make sense on datetimelikes?

[jbrockmendel]

are we actually testing these?

Yes. In particular see the tests.arithmetic tests that are currently xfailed that this un-xfails.

it seems we are enabling all numeric methods; do all of these even make sense on datetimelikes?

DTI/PI get these methods from DTA/PA, which define them with make_invalid_op, so these correctly raise.

[jreback]

ok seems fine to me. @jorisvandenbossche

[jreback]

lgtm. do we need a release note for the additional things that Index can now do? (e.g. can you enumerate them). pls merge master.

[jbrockmendel]

updated+green

[jreback]

thanks @jbrockmendel