{amod} Importing

[asmaloney]

It might make sense to be able to import other files using an import or include statement. These might use a new extension since they won't be complete programs:

Possibilities:

• allow import statements only in the model section and allow anything other than the model section in them (*.amodi)
• allow them per-section (except model) to import just initializers or productions for example (so files don't contain sections) ( *.amodi for initializers, *.amodp for productions?)

Need to consider how it affects readability vs. understanding of the model vs. maintainability.

Related to #5 since that might change the use-case for including initializers separately.