`Colorimeter` interface/device class, for devices that can measure and return XYZ tristimulus values

[JorisVincent]

Class Colorimeter (abstract base), to connect to and use Colorimeters. Should have an abstract method read_XYZ() / measure_XYZ() which returns a measured XYZ tristimulus value.

NOTE: all Colorimeters are by definition also Photometers: if it can return XYZ, it can return Y, which is luminance. Thus, should also implement readLuminance(), as calling read_XYZ and returning only the Y component.

• MockColorimeter (analogous to MockPhotometer) for testing purposes
• An actual concrete Colorimeter class, i.e. Add support for Colorimeter x-Rite i1Pro #37