Export datasets and subsets of datasets

[katjabercic]

• Prepare a dataset to be published to Zenodo, link DOIs,
• (filtered) downloads from the portal in various formats, including helpful code snippets (i.e. for iterating over graphs in Sage, Magma utility code).

Downloads/exports should have settings:

• which columns/properties are included,
• prefix (of length n) or the whole dataset (the latter could be big).