[WIP] Task upload

[ArlindKadra]

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

• Implements task upload.

  1. Generates dictionary that represents object.
  2. Generates xml from the dictionary.

• Adds a function that allows tasks to be created (facilitates task creation for users compared to the task constructor).

• Adds type annotations for the task object.

How should this PR be tested?

New unit tests for different kinds of tasks.

Addresses #657, #656

[codecov-io]

Codecov Report

Merging #607 into develop will decrease coverage by 0.13%.
The diff coverage is 86.91%.

@@ Coverage Diff @@## develop #607 +/- ##
===========================================
- Coverage 90.67% 90.54% -0.14% 
===========================================
Files 36 36 Lines 3690 3743 +53 ===========================================
+ Hits 3346 3389 +43 - Misses 344 354 +10

Impacted Files	Coverage Δ
openml/datasets/dataset.py	88.58% <100%> (+0.03%)	⬆️
openml/tasks/__init__.py	100% <100%> (ø)	⬆️
openml/extensions/sklearn/extension.py	90.62% <64.28%> (-0.25%)	⬇️
openml/runs/functions.py	81.5% <75%> (-0.37%)	⬇️
openml/tasks/functions.py	85.09% <83.33%> (-0.07%)	⬇️
openml/runs/run.py	89.65% <83.33%> (-0.51%)	⬇️
openml/tasks/task.py	94.77% <93.65%> (-1.35%)	⬇️

---

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 f656062...6e70b83. Read the comment docs.

[mfeurer]

Is the estimation procedure common to all task types? If yes, could you please add it to the parent class?

[ArlindKadra]

Nice observation. It also made me realise that the code for some of the tasks can be incomplete since when the class is downloaded from the server it has more information concerning the estimation procedure.

[mfeurer]

Hey, this looks really good! Only a few minor comments. I'll approve once the PEP8 stuff is green.

[mfeurer]

Awesome, I just have a few minor questions/requests, and then we should be good to go with this!