Adds change_slots event to notify UI of slot-modifying changes

Created by: stefanoborini

When the model contains options that might modify the resulting slots, a notification must be issued, so that the UI can take appropriate action.

Provides that mechanism, and documents it.

Fixes #74 (closed)

Created by: codecov[bot]

Codecov Report

Merging #76 into master will decrease coverage by 0.06%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #76      +/-   ##
=========================================
- Coverage   95.47%   95.4%   -0.07%     
=========================================
  Files          49      49              
  Lines         729     762      +33     
  Branches       57      65       +8     
=========================================
+ Hits          696     727      +31     
- Misses         30      31       +1     
- Partials        3       4       +1

Impacted Files	Coverage Δ
force_bdss/data_sources/base_data_source_model.py	100% <100%> (ø)	⬆
...ss/core_plugins/dummy/kpi_adder/kpi_adder_model.py	100% <100%> (ø)	⬆
force_bdss/kpi/base_kpi_calculator_model.py	100% <100%> (ø)	⬆
...plugins/dummy/csv_extractor/csv_extractor_model.py	100% <100%> (ø)	⬆
force_bdss/core_evaluation_driver.py	93.57% <0%> (-0.33%)	⬇

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Created by: stefanoborini

@martinRenou done