Add levels property for MultiIndex

[charlesdong1991]

No description provided.

[codecov-io]

Codecov Report

Merging #973 into master will decrease coverage by 1.48%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #973      +/-   ##
==========================================
- Coverage   94.57%   93.09%   -1.49%     
==========================================
  Files          34       34              
  Lines        6487     6572      +85     
==========================================
- Hits         6135     6118      -17     
- Misses        352      454     +102

Impacted Files	Coverage Δ
databricks/koalas/missing/indexes.py	100% <ø> (ø)	⬆️
databricks/koalas/indexes.py	96.35% <100%> (-0.28%)	⬇️
databricks/koalas/usage_logging/__init__.py	24.54% <0%> (-72.73%)	⬇️
databricks/koalas/usage_logging/usage_logger.py	50% <0%> (-50%)	⬇️
databricks/koalas/__init__.py	78.72% <0%> (-6.65%)	⬇️
databricks/conftest.py	91.48% <0%> (-6.39%)	⬇️
databricks/koalas/frame.py	95.65% <0%> (-0.38%)	⬇️
databricks/koalas/generic.py	95.26% <0%> (-0.31%)	⬇️
databricks/koalas/plot.py	94.04% <0%> (-0.24%)	⬇️
... and 8 more

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[ueshin]

Otherwise, LGTM. (#973 (comment))

[ueshin]

One more thing, let's add a link in docs/source/reference/indexing.rst?

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[ueshin]

LGTM, pending tests.

[ueshin]

Thanks! merging.

[HyukjinKwon]

I had a offline discussion with @ueshin. It requires to collect all data into driver side (similar discussion at #952). Let me take this out in today's release for now.