Refactor workflows and keep workflows alive

[AurelienJaquier]

Separate test and build workflows, and add keep-alive workflow, as was done in BluePyEfe.

[codecov[bot]]

Codecov Report

Merging #245 (98c749b) into master (541636b) will decrease coverage by 0.96%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #245      +/-   ##
==========================================
- Coverage   84.04%   83.08%   -0.97%     
==========================================
  Files          22       22              
  Lines        1147     1123      -24     
==========================================
- Hits          964      933      -31     
- Misses        183      190       +7     

Impacted Files	Coverage Δ
bluepymm/run_combos/calculate_scores.py	73.22% <0.00%> (-5.68%)	:arrow_down:
bluepymm/validate_output/main.py	42.85% <0.00%> (-3.81%)	:arrow_down:
bluepymm/prepare_combos/parse_files.py	61.53% <0.00%> (-0.73%)	:arrow_down:
bluepymm/select_combos/sqlite_io.py	91.66% <0.00%> (-0.65%)	:arrow_down:
bluepymm/prepare_combos/create_mm_sqlite.py	66.66% <0.00%> (-0.49%)	:arrow_down:
bluepymm/main.py	88.88% <0.00%> (-0.40%)	:arrow_down:
bluepymm/select_combos/process_megate_config.py	92.30% <0.00%> (-0.29%)	:arrow_down:
bluepymm/tools.py	77.38% <0.00%> (-0.27%)	:arrow_down:
bluepymm/prepare_combos/main.py	90.19% <0.00%> (-0.19%)	:arrow_down:
bluepymm/select_combos/table_processing.py	85.81% <0.00%> (-0.10%)	:arrow_down:
... and 11 more

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