elastic-tube-1d: Solvers do not support subcycling in implicit coupling

Looking at the FluidSolver.py, I noticed that a few things are a bit uncommon in the time loop:

https://github.com/precice/tutorials/blob/95eea58160af574f8f1ab4ace248d5be53d8e140/elastic-tube-1d/fluid-python/FluidSolver.py#L127-L156

It looks like the checkpointing assumes that the solver time step is equal to the coupling time window. More specifically, whenever preCICE does not ask the solver to read a checkpoint, the solver writes a checkpoint of the current solution. However, this also means that the checkpoint will be updated at every solver time step and the solver will not return to the right state when subcycling.

Two more problems:

• There is no checkpointing of the time itself, we only have a time_it += 1.
• We are essentially tricking preCICE, telling it that we read/wrote checkpoints at different points that we actually do. This makes the code difficult to follow.

The SolidSolver.py, as well as the C++ solvers, also have the same flaw.

Start with the referenced FluidSolver.py time loop at lines 127-156, then compare its checkpointing with SolidSolver.py and the C++ solvers. Reproduce a subcycling case and trace solver time and checkpoint state across coupling windows. Done means all affected solvers restore the correct state without claiming checkpoint operations at points where they did not occur.