Add option --sum_line in project streamlines

[EmmaRenauld]

Quick description

The option to sum lines instead of averaging was already coded in our function but the option was not offered in the script. Added a new arg.

Type of change

Check the relevant options.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Provide data, screenshots, command line to test (if relevant)

...

Checklist

• My code follows the style guidelines of this project (run autopep8)
• I added relevant citations to scripts, modules and functions docstrings and descriptions
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I moved all functions from the script file (except the argparser and main) to scilpy modules
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.59%. Comparing base (31cf4e5) to head (a1ae095).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1346   +/-   ##
=======================================
  Coverage   72.59%   72.59%           
=======================================
  Files         301      301           
  Lines       26283    26285    +2     
  Branches     3700     3700           
=======================================
+ Hits        19080    19082    +2     
+ Misses       5651     5650    -1     
- Partials     1552     1553    +1     

Flag	Coverage Δ
smoketests	69.75% <100.00%> (+<0.01%)	⬆️
unittests	14.21% <0.00%> (-0.01%)	⬇️

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Components	Coverage Δ
Scripts	75.32% <100.00%> (+<0.01%)	⬆️
Library	69.32% <ø> (ø)

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