fix: rel_equal does not work for negative numbers
switch to using rel_equal again; unit tests are unreliable otherwise
refactor: generify RGB (0): generify basic definition
refactor: generify RGB (1): generify non-member operators
refactor: generify RGB (2): trignometric and hyperbolic functions
refactor: generify RGB (3): fix: ldexp missing / basic_rgb-members were still float