Deprecate CPACS related functions

[joergbrech]

Historically, TiXI was intended specifically for handling CPACS files. There are still a few functions in TiXI that are CPACS-specific. Examples for this are tixiAddHeader and tixiAddCpacsHeader. The first one is very old and the second one only works with CPACS < 3.5, see DLR-SL/CPACS#796.

Consequences of removing these functions:

• TiXI as an xml-handling library is not dependent on a specific xml-schema (namely CPACS)
• CPACS users must implement their own tixiAddCpacsHeader, which should be doable with just a few lines in TiXI.

@CLiersch, @MarAlder, what do you think?

[MarAlder]

I have always overlooked this function until now. But I agree with you that this should not be part of TiXI, but part of what I envision as a low-level cpacsLibrary.

I have already practiced this with some users of the community in their corresponding codes, writing the header with the usual createElement methods. Its simple enough.

Instead of updating the function, I would therefore remove it from TiXI.