extend gamma() to incomplete cases
@sgougeon)
Reported by Samuel GOUGEON (BUG DESCRIPTION:
----------------
Extend gamma() to incomplete cases. New proposed syntaxes:
gamma(x, a)
gamma(x, a, "upper") // complementary integral, \int_x^inf
gamma(x, a, b)
gamma(x, a, b, "upper") // complementary integral, \int_x^inf
OTHER INFORMATION:
------------------
* The implementation could use cdfgam().
* gamma() needs unit tests