RegisterAndGetOutputString methods need work

The RegisterAndGetOutputString methods on the following classes need help in some way:

• vtkDelimitedTextWriter
• vtkX3DExporter
• vtkX3DExporterWriter
• vtkGeoJSONWriter
• vtkDataWriter

They are documented to return memory which needs free'd (using delete[]), but the wrappers do not know this, so they end up being leaked if called from any wrapping language.

I think they'd be better off as std::string returns and removal of any memory management shenanigans instead (with new names and deprecations for the existing names since they cannot be changed compatibly).

Cc: @dgobbi