Loading and saving from lattices

[jank324]

🚀 Feature Request

It should be possible to save (and load) lattices as something other than Python code. One idea would be to save to .yaml or something else that has reasonable compatibility.

Motivation

This just makes maintaining lattices easier and removes them from source code.

Suggested by @cr-xu in #4.

[jank324]

Here is a good suggestion to to this in a "more standardised" way: https://github.com/nobeam/latticejson