Base adapter on a fork of the CalculiX repository

[IshaanDesai]

The CalculiX code is available on GitHub https://github.com/Dhondtguido/CalculiX
To bring the adapter closer to CalculiX, it could be put in a fork of the CalculiX repository. Maintaining and updating the adapter could be easier then. But to do this, the changes made to the CalculiX source files need to be applied. The changes are invasive and not trivial to patch, so this is not done currently.

Original idea by @fsimonis

[MakisH]

This adapter could also easily become part of the CalculiX repository, which would make maintenance easier in total: most of the maintenance effort concerns updating to newer CalculiX versions, which is more complicated than the occasional preCICE-related updates. I can eventually work on this in the context of updating the adapter for the latest CalculiX.

If that is not accepted in upstream, fetching and patching the source code could also work. In that case, Coccinelle would probably be very helpful.

Working on a fork would most likely make merging a bit easier, assuming we don't get many merge conflicts. Besides the rewriting of history, we would miss out on a few perks (e.g., we would probably not get automatic formatting, unless the upstream repository also follows the same specification).