Add property: DataFrame.ndim, Index.ndim, MultiIndex.ndim

[itholic]

https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.ndim.html#pandas.DataFrame.ndim

>>> df = ks.range(10)
>>> df.ndim
2

>>> df = ks.DataFrame([[1, 2], [4, 5], [7, 8]],
...                   index=['cobra', 'viper', 'sidewinder'],
...                   columns=['max_speed', 'shield'])
>>> df.ndim
2

[codecov-io]

Codecov Report

Merging #947 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #947      +/-   ##
==========================================
+ Coverage   94.51%   94.53%   +0.02%     
==========================================
  Files          34       34              
  Lines        6449     6464      +15     
==========================================
+ Hits         6095     6111      +16     
+ Misses        354      353       -1

Impacted Files	Coverage Δ
databricks/koalas/series.py	96.22% <ø> (+0.22%)	⬆️
databricks/koalas/missing/indexes.py	100% <ø> (ø)	⬆️
databricks/koalas/missing/frame.py	100% <ø> (ø)	⬆️
databricks/koalas/base.py	94.9% <100%> (+0.06%)	⬆️
databricks/koalas/frame.py	96.03% <100%> (ø)	⬆️
databricks/koalas/missing/series.py	100% <0%> (ø)	⬆️

---

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[ueshin]

@itholic Shall we also add Series.ndim and Index.ndim here?

[itholic]

@ueshin

Thanks for the comment!

i think Series.ndim is already existing.

>>> s
NaN    3.0
NaN    1.0
1.0    2.0
2.0    3.0
3.0    4.0
4.0    NaN
Name: 0, dtype: float64
>>> s.ndim
1

then am i going to only add index.ndim here is okay?

[ueshin]

Maybe we should move this to IndexOpsMixin and remove this in Series?

[itholic]

@ueshin Thanks for the review!

oh i see. i just rearranged them.

[softagram-bot]

Softagram Impact Report for pull/947 (head commit: 918e89a)

⭐ Change Overview

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💡 Insights

• Co-change Alert: You modified frame.py. Often test_dataframe.py (koalas/tests) is modified at the same time.

📄 Full report

• Permalink: Full report for pull/947

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[ueshin]

LGTM, pending tests.