Feature: implement a better prototype trace subset selection function

[danwt]

Context

There is an extremely basic unoptimized version here

https://github.com/informalsystems/modelator-py/blob/bc71f46f0e0cfe85843c63d46c590fce51cbd0fb/modelator/scratch_subset_selection.py

, which could be replaced by a more optimized version using some efficient array encoding, by for example using numpy.

A real long term implementation should probably be done in Rust.