Migrate `.trk` files from .tar files to .zip files.

[willgraf]

The .trk extension just a tarfile (now with gzip compression, thanks to #64). However, there are some compelling reasons to migrate from .tar to .zip:

• Zip files natively support compression, while tarfiles rely on gzip or other methods.
• Zip files support compression of individual files within the container -- this makes several aspects of file handling easier.
• Zip containers are the basis for many common file types, but in particular, npz files (compressed or not) are zip files that can be opened with the zipfile library.

We could potentially do this under the hood and internally check if the .trk is a tar or a zip file and load it appropriately. Both libraries have helper functions to determine this.