Change the calculation precision in mean_rth_order_stat_rmpfr and variance_rth_order_stat_rmpfr to match the n

[bertcarnell]

for n=200, 2^8 bits of precision is required for an exact solution. For larger n, more prevision is required. Either ask the user for the precision or determine it based on the difference between the smallest and largest binomial coefficients in the binomial expansion