Support for non-descendant path in `larvatagger.sh predict`

The larvatagger.sh script requires to be launched from a parent directory of the tracking data files to be automatically labelled with larvatagger.sh predict.

However, users keep trying to run predict from any location on the file system, with the data files on another disk drive.

We need any path to be supported.

EDIT: this actually applies to most switches, not only predict.