[MTR3] British Rail Class 321

This pack includes:

British Rail Class 321/3:

1. Greater Anglia / Varamis Rail Blue doors (16/17m)

Updated content:

1. Removed unused content in mtr_custom_resources.json.

Additional information and known issues:

1. There are no toilets.
2. The train door's position might not being able to align with PSD / APG's positions.
3. Gangways will be rendered between 2 intermediate front cars and might not look like coupled due to mod limitations.