Nikon TIFF channel metadata

[melissalinkert]

@DavidStirling raised that Nikon TIFF channels are not handled as nicely as they could be, particularly when importing into OMERO.

For curated/nikon/carol in particular (and likely others), the fact that the channels are treated as 3 RGB channels and modelled as a single OME-XML Channel with 3 SamplesPerPixel means that the same emission/excitation wavelengths are applied to all 3 channels. On import to OMERO, this means that all 3 channels have the same color by default.

Original metadata indicates that there are different emission wavelengths recorded for each channel, so the better behavior would be to split the channels in the reader so that 3 OME-XML Channels each with 1 SamplesPerPixel are recorded.