In fpdiff.py don't call sys.exit in the middle of functions

It crashes any python code calling fpdiff(..)! Instead return the err code
and exit as normal.

Also: add default args for fpdiff(...) when called from inside python.
Add option in fpdiff(..) to output data to streams other than stdout

* No different when called as a script

* Default is just to use sys.stdout
Add option to disable triangulateio specific parts for dump/restart

Doesn't work with some elements and isn't needed if not using adaptivity.