Improve ns-process-data images when using existing COLMAP model

[jkulhanek]

This PR implements the following:

• Additional flag --skip-images for ns-process-data, which skips copying and downscaling of images. The typical use case is if the user already downscaled the images and wants to run just COLMAP
• Additional parameter --colmap-model-path can be used with --skip-colmap. It allows specifying the path to the COLMAP model as opposed to always using "colmap/sparse/0"
• Fixes a bug with --skip-colmap. When --skip-colmap is used with an existing COLMAP model, the filenames are not renamed (but they are renamed when they are copied and downscaled). This makes the resulting transforms.json contain invalid paths.