Investigate data import in field recipes

[lorenaqendro]

Figure out what what we do with data part of field recipes. In particular take care of the passing around of geographic scope.

When creating modelling fields in the data file we often specify the importer class and datasource that we might have already specified in the containing recipe. This way DC will attempt to import again. We could investigate other ways to do it, considering that we need to keep the geography scope.

[borkurdotnet]

We have implemented the direct overriding of datasources field in recipes (implemented in #321). However the support of nested overriding still needs to be implemented.