Basic structure to allow asserting on tiff files

[tom-seqera]

An initial attempt to provide a scaffold structure to write assertions for tiff files. Eg

then { 
  def tiff = path(process.out.tiff[0]).tiff
  def reference = path('reference.tiff').tiff

  // compare the metadata of the two tiffs, ignoring the bitmap data
  assert tiff.meta == reference.meta

  // or, compare the bitmaps of the two tiffs, ignoring the metadata
  assert tiff.bitmaps == reference.bitmaps
}