Leak memory in stepper

Summary

There is a memory leak in the stepper object.

Steps to reproduce

Run any test with an address sanitizer.

What is the current bug behaviour?

Solver objects will appear as memory leaks.

What is the expected correct behaviour?

No leaks other than the typical standard library appear.

Reproducing input

make build_tests
ctest --output-on-failure

Ideas how to fix this?

The stepper object saves its state with a std::any. Some of these states are shared pointers that must be deleted when stepper objects are deleted.