fix: add standardized SPDX license headers to all Python source files

[joda9]

Closes #56 (file headers / copyright information).

What changed

License headers (all .py files in edisgo/ + setup.py + doc/conf.py)

• Added a uniform #-style copyright and license header to every Python file in the project. Previously most files had no header at all; a few used """...""" module-level docstrings for project info, which caused those strings to appear as module documentation in the auto-generated API reference (sphinx-autoapi renders module docstrings).
• The new header uses # comments so it is invisible to Sphinx, follows the SPDX convention (SPDX-License-Identifier: AGPL-3.0-or-later), and references the git history for contributor attribution instead of hard-coding author names.
• Removed __copyright__, __license__, __url__, and __author__ module-level dunder attributes from edisgo/tools/config.py (the only file that had them).
• Existing genuine module docstrings (e.g. edisgo/io/pypsa_io.py) are kept; only project-info boilerplate """...""" strings were removed.

Pre-commit hook

• Added Lucas-C/pre-commit-hooks (insert-license) to .pre-commit-config.yaml. The hook checks on every commit that the license header is present in all .py files (excluding tests/) and inserts it automatically if missing.
• License text lives in .license-header.txt for easy central updates.

Flake8 fixes (unrelated to headers, found during the same pass)

• edisgo/edisgo.py: wrapped two over-long string literals (E501), removed two unused local variables v_b_load / v_b_feed (F841).
• edisgo/io/generators_import.py: wrapped two over-long comments (E501).
• edisgo/io/mviews_filters.py: SQLAlchemy == None comparisons kept intentionally (generates IS NULL in SQL); suppressed with # noqa: E711.
• edisgo/io/mviews_replacement.py: wrapped long docstring examples (E501), removed unused base_prefix local variable (F841), removed duplicate schema assignment.
• edisgo/io/powermodels_io.py: replaced type(x) == str/dict with isinstance() (E721).
• edisgo/tools/voltage_over_distance.py: removed unused imports Optional, Tuple, get_path_length_to_station (F401); wrapped over-long string literals (E501).

Please include a summary of the change and which issue is fixed.

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• New and adjusted code is formatted using the pre-commit hooks
• New and adjusted code includes type hinting now
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• The Read the Docs documentation is compiling correctly
• If new packages are needed, I added them the setup.py, and if needed the rtd_requirements.txt, the eDisGo_env.yml and the eDisGo_env_dev.yml.
• I have added new features to the corresponding whatsnew file