Feat(Kobotoolbox Loader): Allow data to be added without xls form (in case of update)

[arnaudfnr]

Objective

As a user, I want to be able to update my Kobotoolbox data in a packahe, uploading only the datafile.

Context

All4Trees will update its form data from time to time directly from the website.
The data is xls type with multiple sheets. One question is :

• Should we add the feature in FileLoader so we support any kind of xls files, not only the ones coming from Kobo ?
• In this specific case, should we rather update the Kobotoolbox loader to support a data update, and how should we make the distinction between a non-Kobotoolbox xls and a Kobotoolbox xls ?