[TODO] Support spectral data

[syoyo]

Several ways to support spectral color...

• [x] Single channel TIFF files by encoding wavelength in its filename
  • col_0500_nm.tiff, col_0550_nm.tiff, ...
  • No modification required to TIFF loader/writer
• [x] Multichannel TIFF file
  • TinyDNG supports multichannel TIFF
  • Encode wavelength info to metatata tag?
• [ ] SpectralTIFF: https://www.cg.tuwien.ac.at/research/rendering/spectralTIFF/index.html
• [ ] https://github.com/BeyondRGB/Imaging-Art-beyond-RGB/wiki/TIFF-Output-Images