Added a null check to AVIFormat

In the populateImageMetadata call, if the source metadata has no RAIS
source, the offset is set to 0 instead of throwing a NPE.
Updated ImageMetadata copy constructor

Children of AbstractImageMetadata, when constructed with the intent to
copy another ImageMetadata, will now invoke the zero-param constructor
first to ensure any necessary fields are constructed (to avoid potential
NPEs)
Fixed Metadata translation in SCIFIOFormatWriter

Previously, MetadataRetrieves were only translated to SCIFIO Metadata
during the setId method. Now they are translated during
setMetadataRetrieve calls.

Additionally, the RAIS setting during setId calls was removed, as it
didn't make sense to try and construct a RAIS around the output target
which may or may not exist at that point.