ingest and catalogue FineMath

[oepen]

https://huggingface.co/datasets/HuggingFaceTB/finemath

to get started, i think we only need to 3plus and 4plus parts, which each could go into a separate subdirectory of a new entry, which i suggest to name `finemath/0.0.0/' (seeing as there is no explicit versioning on the HF release).

[jindrahelcl]

(see #51) for comment on the versioning applicable here as well