Add requirements.txt to elastic-tube-1d

[MakisH]

Related to #547 and #563.

I have checked that this works on my system. @BenjaminRodenberg please have a consistency check with anything else related you might be working on at the moment.

FYI @NiklasVin

I need this to proceed with precice/precice#2052

Checklist:

• I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md. -> Not sure if we want to add an entry for every related PR
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.

[BenjaminRodenberg]

Works on my system. I do not have additional requirements for this case at the moment.

[BenjaminRodenberg]

@MakisH do you want to add a changelog entry? I think it's not really required. We will probably add a single entry for the general change in #563. From my perspective: Ready to merge if you are.