Request for ReferencedSeriesUID accessor

[DanielaSchacherer]

As discussed with Chris today, it would add value if - especially for the class highdicom.sr.Comprehensive3DSR - it would be possible to access all the series referenced in the sr with a getter function.

This would eliminate the need to go through the pydicom interface like: sr.CurrentRequestedProcedureEvidenceSequence[0].ReferencedSeriesSequence[0].SeriesInstanceUID.