Saving after connect_as_extension

[LucVV]

As noted on the Zemax forum, an error is currently raised when trying to use OpticStudioSystem.Save after connecting as extension. In this specific case, the private property _OpenFile is kept as None.

I can see two ways to fix this:

1. Set the _OpenFile property directly after connecting as extension
2. Dropping the 'OpenFile' attribute all together and directly checking OpticStudioSystem.SystemFile. As this is a private property, I would not directly consider it a breaking change.

It might be good to address this when addressing #34

[crnh]

I am currently looking into this and am not sure how to handle it.

1. is my preferred option, which can be implemented when we introduce the new connection methods.
2. doesn't seem to be necessary, because OpticStudioSystem.SystemFile is always set (to e.g. <Zemax SAMPLES directory>/Lens.zmx). We can choose to drop the attribute and perform no checks at all.
3. It is also possible to issue a warning if _OpenFile is None, but still save the file.

What do you think?