Allow multiple input file paths

[jfrost-mo]

What problem does your feature request solve?

A single input file path is quite restrictive, for example when loading multiple models, or observation datasets.

Describe the solution you'd like

It would be useful to provide multiple input file paths that are provided to all recipes.

Describe alternatives you've considered

Should this instead just be a variable within the recipe? That would reuse the infrastructure that already exists, and provide additional flexibility.

[jfrost-mo]

It seem to me that the recipe variables are the way to go. We'll still have to consider how to allow display them in rose edit.

[jfrost-mo]

Fixed in #765.