[R/print] update print returns

[JanMarvin]

Initial update to print returns

• Only return useful output
• Wrap all self in invisible(self). We have talked about this in always return invisible? #164 and modernize self$print() for R6 classes #46, but there hasn't been much progress and all the noisy output is really ugly and not helpful at all

As discussed previously: in the future we might want to add something similar to the sheet waiver for output messages. Something like this:

• create Workbook. Output is a workbook
• add a worksheet foo. Output is a workbook \n+ worksheet foo
• add data bar to worksheet foo. Output is a workbook \n+ worksheet foo \n+ data bar
• etc