Enable option to train on huggingface datasets.

[harrykeightley]

Close #11

Features

• Enable training on hf datasets rather than local datasets
• Clean the datasets after they've been converted to a DatasetDict or IterableDatasetDict rather than in a preprocessing stage. This allows cleaning of huggingface datasets.
• Add option not to perform the above cleaning (see do_clean in elpis.models.job.DataArguments)
• Add a second example script hf_example.py for usage with hf datasets.
• Allow building Jobs from dictionaries
• Allow setting a seed.
• Make parameter names on clean_text more general.
• Pave the way for streaming datasets from huggingface instead of having to download them outright. Currently broken but will do in the future if needed. Allow for huggingface dataset streaming #16