[MRMC] Bug in Fox-Glynn algorithm implementation

Dear MRMC developers,

I believe I've found a bug in mrmc. When using transient analysis, MRMC outputs:

>>>>ERROR: Fox-Glynn: lambda >= 25, underflow. The results are UNRELIABLE.

which, I think, is the result of a wrong formula in foxglynn.c, line 199:

k_prime = k_rtp * sqrt_2 + 3.0 / 2.0 * sqrt_lambda;

This line should be:

k_prime = k_rtp * sqrt_2 + 3.0 / (2.0 * sqrt_lambda);

as written in Corollary 3 of the "Computing Poisson Probabilities" paper. Without this line, mrmc always thinks that the right Poisson tail will underflow, writing the warning message.

Amparore Elvio