Improve Bernoulli (LC)CDF Numerical Stability

[andrjohns]

Description

The bernoulli_cdf function directly subtracts the probability parameter from 1, and then multiplicatively accumulates for vectorised inputs:

    const T_partials_return Pi = 1 - theta_vec.val(i);

    P *= Pi;

For small probability (theta) values, this subtraction from 1 will have reduced resolution. When this is not the case, the multiplicative accumulation then carries the risk of underflow.

By moving these calculations to the log scale (i.e., using log1m), the distribution functions will have better resolution across a range of inputs

Current Version:

v4.4.0