Cast data qualities to float

[mfeurer]

Fixes#345.

[codecov-io]

Codecov Report

Merging #353 into develop will increase coverage by 0.21%.
The diff coverage is 100%.

@@ Coverage Diff @@## develop #353 +/- ##
==========================================
+ Coverage 89.69% 89.9% +0.21% 
==========================================
Files 32 32 Lines 2522 2626 +104 ==========================================
+ Hits 2262 2361 +99 - Misses 260 265 +5

Impacted Files	Coverage Δ
openml/datasets/functions.py	91.52% <100%> (+1.47%)	⬆️
openml/datasets/dataset.py	79.57% <100%> (+0.71%)	⬆️
openml/tasks/functions.py	87.55% <0%> (+1.66%)	⬆️

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[amueller]

I'd argue the xsd is wrong? Semantically they are not floats, right?

[amueller]

I'd say this is better then before but still kinda stupid ;)

[mfeurer]

Some qualities are floats, but some aren't. And except for writing down which to cast as int I wouldn't how to do this.