Support for app-only vdb

[prabhu]

Set the environment variable VDB_APP_ONLY=true to use the special app-only vdb for improved performance.

To test locally do:

vdb --clean
export VDB_APP_ONLY=true
export USE_VDB_10Y=true
python depscan/cli.py ...

The new app-only v5 vdb is around 2.1 GB. Therefore, this can work under VMs with < 8 GB memory.

❯ ls -lh /Volumes/Work/vdb
total 2.1G
-rw-r--r-- 1 prabhu 117M Oct 21 01:52 data.index.vdb5
-rw-r--r-- 1 prabhu 1.9G Oct 21 01:52 data.vdb5

The larger 10y app-only vdb is over 8.3 GB. Therefore, a minimum of 10 GB memory is required.

❯ ls -lh /Volumes/Work/vdb_10y
total 8.3G
-rw-r--r-- 1 prabhu 404M Oct 21 05:09 data.index.vdb5
-rw-r--r-- 1 prabhu 7.9G Oct 21 05:09 data.vdb5