[WIP] Unify conversion_utils

[kaminow]

Make it so that constructing models from conversion_utils has the same function signatures regardless of which model you're using. Closes #33.

[codecov-commenter]

Codecov Report

Merging #34 (061037d) into main (011c066) will increase coverage by 10.71%.
Report is 3 commits behind head on main.
The diff coverage is 84.36%.

Additional details and impacted files

[hmacdope]

This all looks good! feel free to merge, then cut release (you can even update conda feedstock now)!