Avoid missing value investment method

[datejada]

Pull request details

Describe the changes made in this pull request

Instead of missing, we add the option none to account for the cases where we don't want any investment variable for that asset at all.

Case study files are updated accordingly.

List of related issues or pull requests

Closes #794

Collaboration confirmation

As a contributor I confirm

• I read and followed the instructions in README.dev.md
• [NA] The documentation is up to date with the changes introduced in this Pull Request (or NA)
• Tests are passing
• Lint is passing

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (b2a73e6) to head (3f7c8b9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #795   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           18        18           
  Lines          904       904           
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  Hits           904       904           

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[gnawin]

Thank you.