Removed difference in KPI vs DataSource - Step 1

Created by: stefanoborini

This PR is the first step in uniform handling of data sources and KPI calculators, as they are pretty much equivalent. The result is that KPI calculators will eventually go away, and we will only have data sources, or a base class and still keep the KPI calculators but just for ease of classification

Created by: codecov[bot]

Codecov Report

Merging #77 into master will decrease coverage by 0.13%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #77      +/-   ##
==========================================
- Coverage   95.47%   95.33%   -0.14%     
==========================================
  Files          49       49              
  Lines         729      708      -21     
  Branches       57       53       -4     
==========================================
- Hits          696      675      -21     
  Misses         30       30              
  Partials        3        3

Impacted Files	Coverage Δ
force_bdss/core_evaluation_driver.py	91.8% <100%> (-2.1%)

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