Adjusting scripts for new assert_inputs_exist function

[karanphil]

Quick description

Here is a small PR adjusting two scripts for the new assert_inputs_exist function. I did not find other scripts where the new feature could simplify the code. Closes #1116

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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)

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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

[arnaudbore]

LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.97%. Comparing base (eb2e9ba) to head (5651a22).
Report is 2 commits behind head on master.

❌ Your project status has failed because the head coverage (69.97%) is below the target coverage (75.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1139   +/-   ##
=======================================
  Coverage   69.96%   69.97%           
=======================================
  Files         448      448           
  Lines       24291    24289    -2     
  Branches     3334     3334           
=======================================
+ Hits        16996    16997    +1     
+ Misses       5874     5872    -2     
+ Partials     1421     1420    -1     

Components	Coverage Δ
Scripts	70.73% <100.00%> (-0.01%)	⬇️
Library	68.97% <ø> (+0.02%)	⬆️