Prepare adapter for full integration

[BenjaminRodenberg]

Goal: inline in the actual ccx code using #ifdefs

Note: I copied this issue from our internal list of todos to make it more visible. Please feel free to directly edit the header, if you can provide more details on the rationale behind this approach. I guess that one reason is that maintaining the Makefile of this adapter is painful, but I'm overview of the CalculiX adapter.

[BenjaminRodenberg]

unifdef might be a handy tool in this context. It allows to easily evaluate ifdefs and inline or remove the code inside of the ifdef-branch. Use-cases I have in mind:

• When working on this issue one might start by checking the diff of the original and adapted CalculiX code and introduce ifdefs step-by-step until the diff is empty.
• Check via a diff on the original CalculiX codebase that no code was added accidentally.
• Set up a test that compares our fork of CalculiX + adapter to the latest CalculiX version. Then we will be able to detect upstream changes easily.