Add possibility to adjust minimum and/or maximum samplesize

[auduno]

It is fairly trivial to add the possibility to set a minimum samplesize (to ensure that post-analysis estimates are better) since this can be simulated as usual. Adding the possibility to set a maximum samplesize, requires the addition of a third threshold which will be used to accept or reject the null-hypothesis when we have reached the maximum samplesize. Finding the third threshold at a precise alpha- and beta-level requires us to implement 3-dimensional gradient descent.