Issue 333 ebi xml gen and sub

This should be implemented. One potential issue here relates to EBI's notion of an `investigation_type` versus our own.  Historically we have tracked investigation_type on a per-study basis, but we actually need to track this on a per-prep basis.  Basically, our notion of a study can have multiple data types (e.g., 16S and metagenomic), but EBI only accepts one type at a time.  So a Qiita study will need to be broken down into several EBI submissions, one per data type, and the accessions will need to be recorded per data type (prep template).