premis conversion should be checked when using buckets

[marly]

Don't generate PiM for PREMIS which will not convert cleanly (specifically when using PiM buckets).

Example: A PREMIS event references a file (but nothing references the event). This event will not have an ADMID reference in the resulting PiM.

[marly]

PREMIS elements which need references:

bitstream objects
representation (non structural) objects
rights
events
agents