fix for scalar PVs not being opened

[kathryn-baker]

Not sure how the tests are supposed to be run so I don't know if they'll pass or not but hopefully they should?

Basically this just adds a check for the scalar type which was being lost before and also raises an error that should be more intuitive to the user as to why the code fails because otherwise it fails somewhat silently by creating the PV but not .open() -ing it.

Also my auto-formatter has changed some other stuff sorry, ignore that!

[MatInGit]

Closing this as we will move to smaller pull requests and reviews on the commit branch