Obtains critical values for the group and continuous sequential MaxSPRT test with Poisson regression data,
using a Wald type upper boundary, which is flat with respect to the likelihood ratio function,
and with a pre-specified upper limit on the sample size.
It is often not possible to select a critical value that corresponds to the exact alpha specified.
Instead, this function will select the least conservative critical value having an alpha below
the one specified, so the sequential analysis is conservative.