[SUB] Rename `pyear` to `planning_horizons` to align conventions with PyPSA-Eur

[tgilon]

Parent Issue (optional)

#132

Affected Component/Workflow

• I have labeled this feature with the affected component(s) using GitHub labels (e.g., SB, CBA, documentation, etc.).

Change Classification

Standard

Security & Data Risk Check

• This feature touches or exposes confidential data.
• This feature adds or changes external data sources, download URLs, APIs, services, credentials, or tokens.
• This feature could affect data integrity or reproducibility of published results.

Risk Level of Feature

N/A

Feature Description / Reason for Change

It is suggested that all the pyear are renamed to planning_horizons to align with PyPSA-Eur convention.

Proposed Implementation / Expected Impact

No response

Implementation Tasks

No response