Improved maintainability

[boris-martin]

To keep clean releases, I thought it might be wise to document somewhere how to maintain everything. In particular, explain how to upgrade to a new CCX version with detailed steps, and what files need to be updated (eg. scripts depending on the version).
Also some minor cleanup along the way :)

Done:

• Added a MAINTAINING.md file.
• Added comments in ccx2.19.c to help locate adapter code more easily
• Added an assertion in the command line arguments parsing to check for bounds.
  Planned:
• Cleanup of the Debian packaging & PaStiX script, to make it easier to update CCX version.

[uekerman]

Documentation is always good 👍
But why here and not on the website? Is it to separate dev docs from user docs? I am not sure if the CalculiX docs is already large enough to force us to do that. We should at least link form the website to this file. Users should see this such that they help maintaining the CalculiX adapter in the long run.

[boris-martin]

I actually planned to ask what was the optimal location once I undrafted this PR, but you were faster :D
I have no hard feelings one way or another. I like having access to doc while offline (if I can access the repo but not the wesbite) but I see the perks of making it accessible to the website. As of now it's short enough to be contained on a single website entry so, why not.
I'll keep it here while i'm drafting it, but it'll be easier to transfer if needed once it's done, so let me know.

[KyleDavisSA]

Is this PR still meant to be a draft, or would it be fine to move along with it?

[boris-martin]

Ideally I'd update after we push the update to CCX 2.20