Fix critical bug that would mis-align port numbers.

When adding new inputs to a TET block, it was possible that the port numbers
would no longer line up between the code block and the outer input ports on
the TET block.  This would cause Simulink models to appear to be hooked up
correctly, but misbehave when actually run.

Fix by ensuring ports are always correct on save.  Both existing ports and
new ports are always numbered to match the internal port numbers on the code
block.  Existing connections are maintained as expected.

Due to this change, it is possible that on saving a TET block connections will
be re-arranged.  Note that these connections will have been incorrect and need
adjustment anyways.