add epsilon term to npv activity coefficients

[Aurashk]

Description

Add a small epsilon value to all activity coefficients in the NPV appraisal optimisation to help prevent spurious investment failures.

Fixes #991

see #998 for a results comparison with simple model

Type of change

• Bug fix (non-breaking change to fix an issue)
• New feature (non-breaking change to add functionality)
• Refactoring (non-breaking, non-functional change to improve maintainability)
• Optimization (non-breaking change to speed up the code)
• Breaking change (whatever its nature)
• Documentation (improve or add documentation)

Key checklist

• All tests pass: $ cargo test
• The documentation builds and looks OK: $ cargo doc

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.12%. Comparing base (c64a7a8) to head (26d18c2).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
...rc/simulation/investment/appraisal/coefficients.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #994      +/-   ##
==========================================
+ Coverage   83.94%   84.12%   +0.17%     
==========================================
  Files          52       52              
  Lines        5794     5844      +50     
  Branches     5794     5844      +50     
==========================================
+ Hits         4864     4916      +52     
+ Misses        695      689       -6     
- Partials      235      239       +4     

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[tsmbland]

Looks good!

[tsmbland]

I would define this as a const at the top of the function. E.g. see

MUSE2/src/input.rs

Line 182 in 29a9a30

const MAX_DISPLAY: usize = 10;

[dalonsoa]

We can also use f64::EPSILON, so it is not an arbitrary small number. See https://doc.rust-lang.org/std/f64/constant.EPSILON.html

[Aurashk]

that's interesting didn't know about that. I changed it to f64::EPSILON * 100.0 because we need it to big enough to force out precision errors. Multiplying by 100.0 is still a bit arbitrary but seems more elegant this way