Fork api

[sahithyaravi]

Reference Issue

#929

What does this PR implement/fix? Explain your changes.

Fork API to clone a dataset.

How should this PR be tested?

id, forked_dataset = fork_dataset(did)
time.sleep(60)
dataset = get_dataset(id)
print(dataset.features)

Any other comments?

For the testing, I have just compared the did. Ideally we want to check if the cloned data==old data and cloned features == old features. But the time taken for the status of the dataset to change to active varies (may be 1 min).
time.sleep(60) should work most of the time, but I am not sure if the tests may fail at some point.
Let me know if it is a good idea to add this to the test. Something like:

old_dataset = get_dataset(did)
id, forked_dataset = fork_dataset(did)
time.sleep(60)
forked_dataset = get_dataset(id)
assertEqual(old_dataset.features == old_dataset.features)

[codecov-commenter]

Codecov Report

Merging #937 into develop will increase coverage by 0.00%.
The diff coverage is 90.00%.

@@           Coverage Diff            @@
##           develop     #937   +/-   ##
========================================
  Coverage    87.76%   87.76%           
========================================
  Files           37       37           
  Lines         4429     4446   +17     
========================================
+ Hits          3887     3902   +15     
- Misses         542      544    +2     

Impacted Files	Coverage Δ
openml/datasets/functions.py	93.85% <90.00%> (-0.27%)	⬇️

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