Add writeStream

[etspaceman]

Solves #232 partially by adding writeStream

[etspaceman]

This still needs a test for withWatermark, I will look into adding that in the near future.

[codecov]

Codecov Report

Merging #480 (01f389d) into master (854d5b6) will increase coverage by 0.27%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #480      +/-   ##
==========================================
+ Coverage   96.23%   96.51%   +0.27%     
==========================================
  Files          60       60              
  Lines        1036     1119      +83     
  Branches        7        8       +1     
==========================================
+ Hits          997     1080      +83     
  Misses         39       39              

Flag	Coverage Δ
2.12.12	96.51% <100.00%> (+0.27%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...c/main/scala/frameless/TypedDatasetForwarded.scala	74.28% <100.00%> (+0.75%)	⬆️
dataset/src/main/scala/frameless/TypedColumn.scala	100.00% <0.00%> (ø)

---

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 854d5b6...01f389d. Read the comment docs.

[imarios]

Thank you for the help @etspaceman! The build I think is failing because of low coverage. It should get green as soon as we add the missing unit tests. I will try to look into the PR today else it may have to wait until after Christmas.

[etspaceman]

No rush! I'll try to get that test in tonight. I'm not quite sure how I'll test it yet but I'll come up with something :D

[etspaceman]

@imarios I was trying to add tests for withWatermark but this is not really possible without the window functions. I think that can be in another PR. So I removed that from this PR

[imarios]

@etspaceman looks great! I am running the tests one more time and merging.