initial hyperparameter listing

[giuseppec]

Adresses #348 still needs documentation and tests.
Is a first working (and hacky) version, which needs to be improved!

@jakob-r , @DanielKuehn87 could you check if your usecases work and make unit tests out of your use cases?
I think we are not uploading default hyperparameters from mlr learners to OpenML and don't know which effect this will have here.

[codecov]

Codecov Report

Merging #378 into master will decrease coverage by 0.07%.
The diff coverage is 92.1%.

@@            Coverage Diff             @@
##           master     #378      +/-   ##
==========================================
- Coverage   96.35%   96.28%   -0.08%     
==========================================
  Files          58       59       +1     
  Lines        2305     2342      +37     
==========================================
+ Hits         2221     2255      +34     
- Misses         84       87       +3

Impacted Files	Coverage Δ
R/helpers.R	97.36% <100%> (+0.04%)	⬆️
R/download.R	88.09% <100%> (ø)	⬆️
R/listOMLSetup.R	91.42% <91.42%> (ø)

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[jakob-r]

Currently all columns are strings including flow.id and setup.id. I wanted to fix it myself but I could not really see how it is done in the other functions. There also seems to be some unused import for type.convert.

[giuseppec]

Hm... I think we simply forgot to use type.convert after we moved from XML to JSON. We use as.integer for columns where we know that they will be integers.
But I would agree to use type.convert in general, I did this a few days ago anyway https://github.com/openml/openml-r/blob/master/R/listOMLStudies.R#L22 . Will open up a separate issue for this.