Add option to use custom file structure

[brendan-m-murphy]

Some additional features discussed on PR #240:

Separate the current data loading logic into parts that

1. get arguments from the model names like we do now,
2. find files given a template and dictionary with all the info to fill the template.

So 1) would do some work to fill the dictionary needed for part 2), but you could use a different file structure if you provide a dictionary will all the info needed to file your template.