Fix: remove non-pip apturl dependency to enable cross-platform setup

[kunalsanga]

This PR fixes a setup blocker where apturl (an Ubuntu-only system package)
was included in requirements.txt, causing installation failures on
Windows and macOS.

Changes:

• Removed apturl from pip requirements
• Added a brief platform note to the README

This improves onboarding and allows new contributors to set up the project
on a clean, cross-platform environment.

Closes #96

[kunalsanga]

Tested on Windows (Python 3.12) in a fresh virtual environment.

[huberthuang0930]

Tested PR #97 on Windows (fresh venv).

• Python: 3.13.5
• Verified apturl==0.5.2 is removed from Grid_based_strong_lensing_for_unsupervised_super_resolution_Anirudh_Shankar/requirements.txt
• Confirmed the original failure reproduces: pip install apturl==0.5.2 -> “No matching distribution found”
• pip install -r requirements.txt now proceeds past the previous apturl blocker (this PR fixes the issue’s root cause)

Note: the requirements file still includes several Ubuntu/Linux-specific packages (e.g., python-apt, systemd-python, Brlapi, etc.) that may still fail on Windows/macOS. That looks like a separate cleanup task from this PR’s scope — I’m happy to open a follow-up issue/PR to split platform-specific deps.