Add bliss main executable

[zhixiangteoh]

Make executable available:

poetry install
poetry shell

Use executable:

# format
bliss -cp {project hydra config path} [{hydra overrides}]
# example
bliss -cp case_studies/summer_template 'mode=generate'

Fixes #751.

[jeff-regier]

Looks good to me

[codecov]

Codecov Report

Merging #756 (9505834) into master (fb845d9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #756   +/-   ##
=======================================
  Coverage   88.48%   88.48%           
=======================================
  Files          14       14           
  Lines        1259     1259           
=======================================
  Hits         1114     1114           
  Misses        145      145           

Flag	Coverage Δ
unittests	88.48% <ø> (ø)

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