Added simple arithmatic operators to artist::colour and a test

[bruno-j-nicoletti]

Added +, - and * operators to artist::colour, along with a simple test.

[djowel]

Thank you @bruno-j-nicoletti . Looks good to me!

[bruno-j-nicoletti]

I was tempted to add a float& operator[](int index) to artist::color as well, along with a begin/end so you can treat a colour as in iterable object. There would be both const and non const versions of those. Not that pressing though.

[djowel]

I was tempted to add a float& operator[](int index) to artist::color as well, along with a begin/end so you can treat a colour as in iterable object. There would be both const and non const versions of those. Not that pressing though.

Nah. It takes discipline to not be carried away. I'm glad you were not tempted.