Add downsampling step

**Describe the change**
@loforest asked to have an additional, final step to the algorithm that ensures that the list of final cloud layer (bases) are separated by a specific amount (specifiable by the user as a function of altitude). Layers too close from one another should get merged in that step.

No file, test, or entry point is named in the issue. Start by locating the algorithm's final cloud-layer/base processing in the Python package and trace how altitude-dependent user settings are represented. Done means a final downsampling step merges layers closer than the specified altitude-dependent separation.

Written by the indexing model from the issue text.