Saving a document frees resources and makes it invalid for further editing

[joergbrech]

saveDocument frees resources and sets the document status to SAVED.

https://github.com/DLR-SC/tixi/blob/470570de2969bbe50d8c4179316fab7af430d5e6/src/tixiInternal.c#L1233-L1237

As a consequence, a saved document cannot be edited anymore:

https://github.com/DLR-SC/tixi/blob/470570de2969bbe50d8c4179316fab7af430d5e6/src/tixiImpl.c#L1231-L1234

This is counter-intuitive, because there is also tixiCloseDocument and having intermediate saves in a long simulation run is a valid use case.

Thanks @cliersch for pointing this out.