implement normalization constant for discrete truncated power law

The directly computable normalization constant significantly reduces
the runtime for fitting a discrete truncated power law.

For a discrete truncated power law of form f(x) = x^-a*e^(-l*x), the
normalization constant C such that that sum of C*f(x) from x=xmin to
x=Infinity equals 1 is
   C = e^(l*xmin) / Phi(e^-l, a, xmin),
where Phi is the Lerch transcendent (aka Lerch Phi) function.