Fix checkpointing and subcycling for solid.py

[BenjaminRodenberg]

Fixes checkpointing by considering u, v, and a. Closes #50.

Note: This fix requires precice/fenics-adapter#170 which is currently not merged and also not released in v2.0.0 of fenics-adapter. Doing a 2.1.0 release of the adapter for this feature would not be a big problem from my perspective. Adding this fixed version of the tutorial to the distribution release gives us another example using subcycling + time interpolation.

[MakisH]

I see several changes here. Which one is the essence of the fix? Why was it broken before? Edit: I understand the issue was that the checkpoint was not complete, hence now precice.store_checkpoint((u_n, v_n, a_n), t, n).

Note: This fix requires precice/fenics-adapter#170 which is currently not merged and also not released in v2.0.0 of fenics-adapter. Doing a 2.1.0 release of the adapter for this feature would not be a big problem from my perspective.

In the distribution, we need that all tutorials work with the respective versions of components released in the same distribution. So yes, please do that, if needed.

[MakisH]

From my limited viewpoint, this is ready to merge as soon as the corresponding FEniCS adapter feature is available.

@IshaanDesai please still review for the FEniCS-related details.

[IshaanDesai]

Looks good 👍 I have only two minor comments.