let split node specify property to split on, and join auto join correctly

[dceejay]

or manually the specified one.

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

This PR lets the split node specify the property that it will split on... it adds the specified property to the parts property as 'parts.property'. It defaults to payload so should be non-breaking. So far testing and extra tests all work OK.
The join node can then rejoin the specified property automatically. Or operate on the property configured - again defaults to msg.payload.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run npm run test to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality