add ciexyz_to_cielab, cielab_to_xiexyz and delta_e + higher precision in cie xyz <-> linear rgb

Signed-off-by: Mattias Andrée <maandree@operamail.com>
info: doc ciexyz_to_cielab cielab_to_xiexyz delta_e

Signed-off-by: Mattias Andrée <maandree@operamail.com>
buf fix

Signed-off-by: Mattias Andrée <maandree@operamail.com>
info: info on jnd

Signed-off-by: Mattias Andrée <maandree@operamail.com>