Export datasets and subsets of datasets

- 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). 