Allow KPIs to be specified as datasource outputs.

Created by: stefanoborini

This PR removes the KPI Calculator infrastructure completely, and replaces it with the possibility to specify which outputs of the DataSource are KPIs. They will be rendered as checkboxes in the UI.

This PR breaks both the UI and the external plugins.

Created by: codecov[bot]

Codecov Report

Merging #120 into master will decrease coverage by 0.57%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #120      +/-   ##
==========================================
- Coverage      94%   93.42%   -0.58%     
==========================================
  Files          39       35       -4     
  Lines         800      715      -85     
  Branches       62       58       -4     
==========================================
- Hits          752      668      -84     
  Misses         40       40              
+ Partials        8        7       -1

Impacted Files	Coverage Δ
force_bdss/io/workflow_writer.py	88.88% <ø> (ø)
force_bdss/io/workflow_reader.py	100% <ø> (ø)
force_bdss/api.py	100% <ø> (ø)
force_bdss/base_extension_plugin.py	100% <ø> (ø)
force_bdss/core/workflow.py	100% <ø> (ø)
force_bdss/core_evaluation_driver.py	86.58% <100%> (+0.16%)
force_bdss/core_mco_driver.py	91.04% <100%> (+0.13%)

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