ENH: Add DataFrame/Series.align

[sinhrks]

df1 = pd.DataFrame({'A': np.random.randn(5), 
                   'B': np.random.randn(5)},
                  index=list('ABCDE'))
ddf1 = dd.from_pandas(df1, 2)

df2 = pd.DataFrame({'A': np.random.randn(5), 
                   'B': np.random.randn(5)},
                  index=list('BDEFG'))
ddf2 = dd.from_pandas(df2, 2)

res1, res2 = ddf1.align(ddf2)
res1.compute()
#            A         B
# A  -0.693455  1.729368
# B   1.486508 -0.489367
# ..       ...       ...
# F        NaN       NaN
# G        NaN       NaN
# 
# [7 rows x 2 columns]

res2.compute()
#            A         B
# A        NaN       NaN
# B   0.191631  0.023919
# ..       ...       ...
# F   0.010644 -0.366820
# G  -0.651027 -0.126170
# 
# [7 rows x 2 columns]

[mrocklin]

I don't have any experience with pandas.align, so I'm not sure that I'm a good judge here, but the test suite and implementation here seems good to me. +1

[jreback]

align is used all over the place internally in pandas esp for join/merge

not sure if that impacts

[mrocklin]

This has merge conflicts from recently merged PRs.