fix typo
add (empty) PlotUnit
use uint for canvas size
add cie1931 colour space
implement PlotUnit::clear
Vector3 is a good representation of a tristimulus
implement PlotUnit::plot
implement PlotUnit::plot_pixel

The implementation uses a macro. I do not know whether this is the best way.
remove redundant parentheses around if condition
disable dead code warnings
fix typo in PlotUnit