posm: positronium continuum

Positronium continuum (Brown & Leventhal 1987)

$$A(E) = K{2\over{(\pi^2-9)E_c}} [{E(E_c-E)\over{(2E_c-E)^2}} ... ...er{(2E_c-E)^3}}\log({{E_c-E}\over{E_c}}) + {(2E_c-E)\over{E}}]$$

for $E < E_c$ = 511 keV, where:

norm	$K$, normalization