Storing public data sets in installation directory is problematic

Downloading the public data sets and store them in the besca installation directory may be a problem if the downloading user does not have permissions to wirte into the installation directory, for example, if the besca installation is shared or if it is within a docker container.

see functions https://github.com/bedapub/besca/blob/4c7f7f1a92de2bc6c61b177e8fe6d0d16bb23d1d/besca/datasets/_datasets.py

workaround: provide option so that the user can specify an alternative path to the besca installation directory.

Start in besca/datasets/_datasets.py and inspect the functions that download and store the public data sets. Add a user-selectable alternative storage path, then verify that downloads work when the Besca installation directory is not writable, including in a shared installation or Docker container.

Written by the indexing model from the issue text.