More Backends

[mllg]

• Includes a DataBackend for Matrix
• Backend construction helper: as_data_backend()
• New field "formats" for DataBackend which describes the possible output formats
• We need to define how auto-conversion of formats can be handled

Matrix backend is still experimental, we need to progress with the pipelines before I can finalize this.

[codecov]

Codecov Report

Merging #65 into master will increase coverage by 0.03%.
The diff coverage is 90%.

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
+ Coverage   90.64%   90.67%   +0.03%     
==========================================
  Files          60       61       +1     
  Lines        1293     1341      +48     
==========================================
+ Hits         1172     1216      +44     
- Misses        121      125       +4

Impacted Files	Coverage Δ
R/TaskClassif.R	86.95% <ø> (ø)	⬆️
R/mlr_tasks.R	90.9% <ø> (-9.1%)	⬇️
R/TaskRegr.R	100% <ø> (ø)	⬆️
R/DataBackend.R	100% <100%> (ø)	⬆️
R/DataBackendDataTable.R	100% <100%> (ø)	⬆️
R/Task.R	87.01% <100%> (ø)	⬆️
R/helper_misc.R	94.73% <100%> (+0.61%)	⬆️
R/DataBackendOverwrite.R	95.65% <50%> (+0.19%)	⬆️
R/DataBackendRbind.R	91.66% <50%> (+0.36%)	⬆️
R/DataBackendCbind.R	94.73% <50%> (+0.29%)	⬆️
... and 4 more

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