Add sde command for flattening parent+submodels into a single mdl file

[chrispcampbell]

Recent versions of Vensim have supported the concept of submodels. Vensim offers a command (File > Save parent and submodels merged) to export the combined files as a single, flattened mdl file.

With a small layer on top of the existing preprocessor, SDEverywhere should also be able to do this flattening step, which offers two advantages:

• allows for extra validation checks (e.g. detect duplicate or differing definitions)
• more cloud friendly (can run in GitHub Actions or other remote CI environments)

This will be considered an experimental feature at first, and we will evaluate its usefulness over time.