Add support for GroupBy.{cumcount,cummin,cummax,cumprod,cumsum}

[wjsi]

What do these changes do?

Add support for GroupBy.{cumcount,cummin,cummax,cumprod,cumsum}

Related issue number

Fixes #1068

[codecov]

Codecov Report

Merging #1069 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1069      +/-   ##
==========================================
+ Coverage    92.8%   92.87%   +0.07%     
==========================================
  Files         622      626       +4     
  Lines       47700    48182     +482     
  Branches     7058     7112      +54     
==========================================
+ Hits        44267    44751     +484     
+ Misses       2285     2276       -9     
- Partials     1148     1155       +7

Impacted Files	Coverage Δ
mars/operands.py	93.58% <100%> (+0.01%)	⬆️
mars/dataframe/groupby/cum.py	100% <100%> (ø)
mars/dataframe/groupby/apply.py	94.91% <100%> (-0.05%)	⬇️
mars/dataframe/groupby/__init__.py	100% <100%> (ø)	⬆️
mars/web/session.py	74.91% <0%> (-1.38%)	⬇️
mars/scheduler/session.py	96.44% <0%> (-1.19%)	⬇️
mars/scheduler/operands/base.py	94.56% <0%> (-1.09%)	⬇️
mars/scheduler/operands/shuffle.py	91.2% <0%> (-0.8%)	⬇️
mars/worker/storage/core.py	93.14% <0%> (-0.41%)	⬇️
mars/dataframe/base/__init__.py	100% <0%> (ø)	⬆️
... and 14 more

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[qinxuye]

LGTM