dgamma(a,b): Gamma distribution with parameter a and b.
  f(x)= b^a/( Gamma(a)) x^(a-1) e^-(b*x)
  mean=a/b
  variance=a/b^2

dgamma(x,shape=a,scale=s): Gamma distribution with parameter a and s.
  f(x)= 1/(s^a Gamma(a)) x^(a-1) e^-(x/s)
  mean=a*s
  variance=a*s^2