This PR adds automatic download and caching functionality for vocabulary
trees. Sample output:
```
$ colmap vocab_tree_matcher --database_path ~/data/south-building/database.db
I20241212 18:17:51.708911 427449 misc.cc:44]
==============================================================================
Feature matching
==============================================================================
I20241212 18:17:51.724920 427450 sift.cc:1426] Creating SIFT GPU feature matcher
I20241212 18:17:51.755941 427449 pairing.cc:234] Generating image pairs with vocabulary tree...
I20241212 18:17:51.756253 427449 file.cc:356] Downloading file from: https://github.com/colmap/colmap/releases/download/3.11.1/vocab_tree_flickr100K_words256K.bin
I20241212 18:17:56.891381 427449 file.cc:361] Caching file at: "/Users/jsch/.cache/colmap/vocab_tree_flickr100K_words256K.bin"
I20241212 18:17:57.423804 427449 pairing.cc:355] Indexing image [1/128]
...
$ colmap vocab_tree_matcher --database_path ~/data/south-building/database.db
I20241212 18:18:02.601915 427813 misc.cc:44]
==============================================================================
Feature matching
==============================================================================
I20241212 18:18:02.614478 427814 sift.cc:1426] Creating SIFT GPU feature matcher
I20241212 18:18:02.639487 427813 pairing.cc:234] Generating image pairs with vocabulary tree...
I20241212 18:18:03.135216 427813 pairing.cc:355] Indexing image [1/128]
...
```