bc.export.raw_to_mtx should check for row orders between barcodes and metadata

[ajulienla]

bc.export.raw_to_mtx( adata, outpath = '', write_metadata=True)

write files that are not uploadable in the mongodb immediately as:

ERROR: The cell order in files
*metadata.tsv
 *barcodes.tsv

ARE NOT identical.

This should be check at this step to facilitate public dataset integration and export