feat: retain common samples on pgx multiomics upload

[ESCRI11]

As discussed, retain the common samples when uploading multi omics data from pgx files.

[zitoa]

if common_rows is NULL (or length==0) should we display alert and abort the session? @ESCRI11

[ESCRI11]

@zitoa that should be handled by the already placed checks on samples step -- let me double check

[ESCRI11]

@zitoa Even better, its already handled by existant code; we were checking samples match but not auto-loading them. so all good

[ivokwee]

For the moment this is fine. However in the future we want also to be able to handle not matched multi-omics datasets. But in that case we need maybe a different data structure.