Start implementing core features to access old revisions of a series

EPIC: #519

Goals:

• web API can list revisions of either provider/dataset/series
• web API can view a specific revision of either provider/dataset/series
• clients can add a revision parameter to the fetch functions
• user can list revisions of a dataset or series on the website - user can view and download (XLS, CSV...) a specific revision of a dataset or series on the web site

Tasks

First step is to implement the feature in the data model layer, so that it can be ran manually via a script – before adding it to the API and website.

Implement in dbnomics-data-model:

• access list of revisions of a series
• access past revision of a series get_series(series_id, revision_id=<commit_id>) (optional: by date)
• access list of revisions of a dataset
• access past revision of a dataset metadata (think about it and add precision to the task list...)
• create a CLI script like find_series_at_revision.py <series_id> <revision_id> (and another one for datasets, or a script with more options...) returning JSON/TSV data depending on the requested data

Later:

• propose new API endpoints or parameters
• add revision features to data storage layer
• add revision features to web API
• add revision features to Python client
• add revision features to web site

Extra:

• check if the time series editor can load a time series at revision