Adding support for list of lists of paths in assert_inputs_exist

[karanphil]

Quick description

I added support for a list of lists of paths in scil_assert_inputs_exist.py. This arises when you mix nargs='+' with action='append'.

Closes #1067

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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

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 69.46%. Comparing base (ae1ce8f) to head (2aeb19b).
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1086      +/-   ##
==========================================
+ Coverage   69.45%   69.46%   +0.01%     
==========================================
  Files         447      447              
  Lines       24052    24058       +6     
  Branches     3291     3295       +4     
==========================================
+ Hits        16706    16713       +7     
+ Misses       5947     5945       -2     
- Partials     1399     1400       +1     

Components	Coverage Δ
Scripts	70.26% <ø> (+0.01%)	⬆️
Library	68.40% <25.00%> (+<0.01%)	⬆️