Feature Request: expose recipe/package paths

[JSKenyon]

As discussed in today's meeting, in order to make recipes more portable, it would be useful to specify paths relative to the recipe definition/some imported package.

This came up while I was attempting to set up a flagging subrecipe which requires a non-stimela yaml file as input. There is (at present) no easy way to tell stimela were the additional yaml file lives relative to the recipe.