Fixed plane count when writing tiffs

Only sizeT planes were being written when saving tiffs, because the tiff
format doesn't seem to have metadata for sizeC and sizeZ (they are
always 1, unless RGB.. then sizeC = 3).

So when translating to TIFFFormat, # plane IFDs are now created instead
of sizeT, effectively squashing all dimensions to T.

It would be nice to have another solution in TIFF but I don't think it's
possible without writing invalid tiff files. That's what OME-TIFF is
for.