Add 3D case support capability for serial FSI cases

[IshaanDesai]

This PR adds 3D case handling capability to the adapter. Currently only 3D FSI serial cases are tested. The testing is done with the help of the elastic-tube-3d case: precice/tutorials#222. Closes #1

[precice-bot]

This pull request has been mentioned on preCICE Forum on Discourse. There might be relevant details there:

https://precice.discourse.group/t/does-the-fenics-adapter-support-3d-case/696/2

[BenjaminRodenberg]

Some comments mainly on code quality. I did not try to run the code.

General comment: We also need tests for 3d. This is even more important than for 2d, because I expect that the 3d feature will not be used as frequently as 2d and we still want to be sure that it does not break over time.

[BenjaminRodenberg]

Overall looks good and I think it was really helpful to split this larger feature up into smaller PRs. There are actually only a few changes here.

I still have to check the sibling case precice/tutorials#222.

Only critical point I see is that Expressions are not supported yet. I understand that handling expressions correctly in 3D is tricky, because the way how we are constructing the interpolant is mostly manual and therefore implementing this in 3D is challenging. Maybe this is also a good hint that we should start looking for a more general strategy, such as #119.