move the concept check to template arg list (below for details)

R. Martinho Fernandez, 'Almost Static If': http://flamingdangerzone.com/cxx11/2012/06/01/almost-static-if.html
generify power, error, gamma, rounding and remainder
generify remaining functions
revive function declarations as a means of self-documentation
more tweaks for readability
liberate cmath from rgb