Introduce InternalFrame.index_level.

[ueshin]

Currently there are different ways to check the number of the index columns. We should introduce InternalFrame.index_level and use it.

[codecov-io]

Codecov Report

Merging #1890 (62f413f) into master (5a7c2fc) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1890      +/-   ##
==========================================
- Coverage   94.17%   94.13%   -0.04%     
==========================================
  Files          40       40              
  Lines        9957     9960       +3     
==========================================
- Hits         9377     9376       -1     
- Misses        580      584       +4     

Impacted Files	Coverage Δ
databricks/koalas/accessors.py	93.03% <ø> (ø)
databricks/koalas/frame.py	96.69% <100.00%> (-0.05%)	⬇️
databricks/koalas/groupby.py	91.47% <100.00%> (ø)
databricks/koalas/indexes.py	96.82% <100.00%> (ø)
databricks/koalas/indexing.py	92.76% <100.00%> (ø)
databricks/koalas/internal.py	96.50% <100.00%> (+0.03%)	⬆️
databricks/koalas/series.py	96.86% <100.00%> (-0.10%)	⬇️
databricks/koalas/generic.py	93.62% <0.00%> (-0.29%)	⬇️
databricks/koalas/namespace.py	83.08% <0.00%> (-0.21%)	⬇️
... and 1 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5a7c2fc...62f413f. Read the comment docs.

[ueshin]

Thanks! I'd merge this now.