Clean up project data

[martinherrerias]

From: #202 (comment), related to #216

The repo currently includes a bunch of example data used for the demos (and studies?). We should try to weed out:

• (Small) datasets that are useful for testing and should be bundled with the package. Anything bigger than a few kB should probably be zipped and tracked as binary.
• Demo/study-specific datasets, that could be stored outside (e.g. Zenodo) and downloaded at runtime.