Add function that exposes a fold Cache

[jjti]

Useful for things like primer design where the user is going to query many possible start + end positions, checking each for a high delta g, ie low foldability

[jjti]

done with https://github.com/Lattice-Automation/seqfold/commit/4cfcdb03eb48ef341532ce0d2fec6d6b942a0ce9