To be removed

a list of things that I think should be removed from the package:

- [ ] module `bgflow/nn/flow/circular.py`
- [ ] module `bgflow/nn/flow/checkerboard.py`
- [ ] module `bgflow/nn/flow/kronecker.py`
- [ ] module `bgflow/utils/internal_coordinates.py`
- [ ] module `bgflow/nn/flow/crd_transform/_deprecated_ic.py`
- [ ] subpackage `bgflow/distribution/sampling/_mcmc`