Add total_loss parameter for fiber attenuation (OTDR measurement support)

[LynchXLQ]

Summary

This PR adds support for specifying fiber attenuation as total loss (dB) instead of only via loss_coef (dB/km). This addresses the use case where users have OTDR measurements that report total span loss directly.

Resolves #441

Changes

gnpy/core/parameters.py (FiberParams)

• Accept total_loss as an alternative to loss_coef
• When total_loss is provided, derive loss_coef = total_loss / length
• Support both scalar and frequency-dependent total_loss (dict with value and frequency keys)
• Fully backward compatible: loss_coef continues to work unchanged
• asdict() preserves the original parameter (total_loss or loss_coef)

gnpy/core/elements.py (Fiber)

• to_json exports total_loss / total_loss_per_frequency when total_loss was the input

gnpy/tools/yang_convert_utils.py & convert_legacy_yang.py

• YANG format conversion support for total_loss / total_loss_per_frequency

Tests

• Scalar total_loss → correct loss_coef derivation
• Frequency-dependent total_loss → correct per-frequency loss_coef
• Backward compatibility (loss_coef still works)
• Round-trip serialization via asdict()

Usage Example

{
  "uid": "fiber-span-1",
  "type": "Fiber",
  "params": {
    "length": 80,
    "length_units": "km",
    "total_loss": 16.0,
    "con_in": 0.5,
    "con_out": 0.5,
    "pmd_coef": 1.265e-15
  }
}

cc @jktjkt

[EstherLerouzic]

Dear @LynchXLQ , thanks a lot for your contribution! We will review it and will give you feedback as soon as possible. We normally use gerrithub for review (https://review.gerrithub.io/q/project:Telecominfraproject/oopt-gnpy). If you are familiar with it please push your commits there. If not I can do it for you, so that we can start the review process.
Regards

[LynchXLQ]

Thanks @EstherLerouzic! I've pushed the changes to GerritHub: https://review.gerrithub.io/c/Telecominfraproject/oopt-gnpy/+/1231186

Looking forward to your feedback!