Fixing issue with reaonly filesystems

[RossHammer]

Type of PR

• BUG
• FEAT
• MAINT
• DOC

Is your Pull Request linked to an existing Issue or Pull Request?

closes #2356

Give a brief description for the solution you have provided

I have moved creating the data cache to be when the data is requested rather than on import. This will allow anyone using this on a readonly filesystem to be able to import an use the package.

PR Checklist

• Added documentation for changes
• Added feature to example notebooks or tutorial (if appropriate)
• Added tests (if appropriate)
• Updated CHANGELOG.md (if appropriate)
• Made changes based off the latest version of Splink
• Run the linter
• Run the spellchecker (if appropriate)

[RobinL]

Thanks! Looks reasonable to me, but Andy was the original author of this code will get his opinion first.. Might also solve #2341 @ADBond look ok to you?

[ADBond]

Yep, this looks great, thanks!