Toniof auxdata provision

[TonioF]

This is a change to make the Prior Engine run on DIASes. It uses an AuxDataProvider. An AuxDataProvider implements two methods:

• listElements(folder, pattern): Retrieves a list of all files in a folder (that optionally may match a pattern)
• assure_element_provided(file_name): Ensures that the file is actually located at the location
  These are bascially oneliners for the Default Implementation, which just checks a directory in the file system. On a DIAS system, also the content of a bucket is changed and assure_element_provided might trigger a download.
  To run this, you will need the latest version of multiply_core (master branch).