Adding inflation rate to numpy NPV calc

[johnjasa]

Allow user to provide inflation rate to NumPy Financial NPV model

Resolves #339.

1. Added an optional inflation_rate input to NumpyFinancialNPVFinanceConfig. Defaults to 0.0, validator-constrained to [0, 1].
2. Combined it with discount_rate via the Fisher equation (1 + r_eff) = (1 + discount_rate) * (1 + inflation_rate) to match how ProFAST combines its real discount rate and general_inflation inputs, keeping the two finance backends consistent.
3. Default behavior is preserved: with inflation_rate=0 the effective rate reduces to discount_rate, so existing configs and tests don't change.

Section 1: Type of Contribution

• Feature Enhancement
  • Framework
  • New Model
  • Updated Model
  • Tools/Utilities
  • Other (please describe):
• Bug Fix
• Documentation Update
• CI Changes
• Other (please describe):

Section 2: Draft PR Checklist

• Open draft PR
• Describe the feature that will be added
• Fill out TODO list steps
• Describe requested feedback from reviewers on draft PR
• Complete Section 8: New Model Checklist (if applicable)

Type of Reviewer Feedback Requested (on Draft PR)

Structural feedback:

• The original issue suggested summing the two rates. I switched to the Fisher equation (1 + r_eff) = (1 + discount_rate) * (1 + inflation_rate) because it's the exact relationship between real and nominal rates and is what ProFAST uses internally.

Implementation feedback:

• inflation_rate is validated with range_val(0, 1) to mirror discount_rate. Let me know if a different range (e.g. negative inflation) should be allowed.

Other feedback:

• N/A

Section 3: General PR Checklist

• PR description thoroughly describes the new feature, bug fix, etc.
• Added tests for new functionality or bug fixes
• Tests pass (If not, and this is expected, please elaborate in the Section 6: Test Results)
• Documentation
  • Docstrings are up-to-date
  • Related docs/ files are up-to-date, or added when necessary
  • Documentation has been rebuilt successfully
  • Examples have been updated (if applicable)
• CHANGELOG.md
  • At least one complete sentence has been provided to describe the changes made in this PR
  • After the above, a hyperlink has been provided to the PR using the following format:
    "A complete thought. PR XYZ", where
    XYZ should be replaced with the actual number.

Section 4: Related Issues

Fully resolves #339.

Section 5: Impacted Areas of the Software

Section 5.1: New Files

• N/A

Section 5.2: Modified Files

• h2integrate/finances/numpy_financial_npv.py
• h2integrate/finances/test/test_numpy_financial_npv.py

Section 6: Additional Supporting Information

N/A

For typical project finance rates (real 5%, inflation 4%), the multiplicative form gives r_eff = 0.092 vs. the additive 0.09 results in a 0.2 pp difference.

Section 7: Test Results, if applicable

$ python -m pytest h2integrate/finances/test/test_numpy_financial_npv.py -v
...
5 passed, 8 subtests passed in 0.37s

Ruff lint + format clean on the two modified files.

Section 8 (Optional): New Model Checklist

N/A