Two-stage rocket ascent needs fixing

[sbcowan]

The two-stage rocket ascent example returns an error now.

Problems:

• There is a difference in behaviour between the .py and .ipynb file.
• Compared to before the thrust refactoring, there is now an error for an exceeded minimum step size.

The question of what exactly needs to be shown with this example needs answering. Depending on that answer, the problem with the example is more or less problematic.

The problem occurs since the thrust refactoring in two_stage_rocket_ascent.py of commit 730e6ac.

I tested a few commits:

• With commit 730e6ac (random old commit without thrust refactoring or integrator/propagator refactor), the stage separation occurs after 8.33 minutes with no errors. This is run with tudatpy=0.6.4 and tudat=2.10.6.
• With commit 0ebc2e8 (commit where Miguel merged among others the updated two stage rocket ascent example for the .py file), the stage separation occurs after 7.75 minutes with a propagation error due to the minimum step size being exceeded. This commit has the updated thrust refactor, without the integrator/propagator refactor. This was run with tudatpy=0.7.1 and tudat=2.11.1.
• With commit f8989ee (master), which includes the thrust refactor and the integrator/propagator refactor, the stage separation also occurs after 7.75 minutes with a propagation error due to the minimum step size being exceeded. This is also with tudatpy=0.7.1 and tudat=2.11.1.

This is all for the .py files. The .ipynb files were not updated in sync with these commits. Running the .ipynb file of commit f8989ee that I refactored results in an index error because there are too few time steps to create results (because of the premature termination). Here the stage separation already occurs after 3.91 minutes.