Replace anaconda-project downloads

[maximlt]

anaconda-project has a handy feature that allows to declare a series of files to download (and optionally unzip) when preparing a project (see https://anaconda-project.readthedocs.io/en/latest/user-guide/reference.html#file-downloads). Some day we will need to replace anaconda-project by another tool (e.g. conda-project, pixi) which, at the moment, don't provide this feature. To prepare this transition, we'll need to find an alternative way to download data.

Features we use:

• Unzip
• Declare the filename to save as, or folder (filename: data) for archives to unzip
• No re-download when the file is found in its target location => We rely on this feature for testing purposes, moving test data to the right place before preparing the project

Potential alternatives:

• pooch (https://www.fatiando.org/pooch/latest/)
• TODO: look for more!

[maximlt]

Noting that the ability to download data while preparing the project/environment is useful for instance when an example is deployed. Otherwise, if an example requires a large dataset to be downloaded, the first user is going to have to wait a little too long :) No big deal but not great.