add non rgb 3b dataset test

[vincentsarago]

update:

• In this case, the photometric interpretation stored in the underlying TIFF is still RGB, but GDAL writes a custom photometric intepretation in the custom GDAL_METADATA XML tag.
• This means we would have to change the implementation of both .photometric and .colorinterp based on the GDAL_METADATA

We'll need to decide how closely we want to match GDAL semantics. For now, we'll put this on hold until someone comes along with an opinion