Splink datasets revamp

[ADBond]

Some changes would be useful for splink_datasets

A couple of changes to make:

• would be nice for the different attributes to be statically available - probably don't need quite as much magic is at currently has
• currently this won't work if the process doesn't have write permissions (see Splink install failing due to splink_datasets PermissionError #2341, and fix in Fixing issue with reaonly filesystems #2357). Might be able to find some happy ground where at least some partial functionality works, either retrieving data straight into memory, or (partially) storing on the object if caching is not available