Implement stratified sample size method from Bernstein and Lagakos

[nanxstats]

From Keaven: The Bernstein and Lagakos method¹ can be added to gsDesign2 as an alternate stratified sample size method for comparison².

Keaven has kindly agreed to work with Yujie on the spec first (function name, input arguments, output structure) to align with the gsDesign2 style guide.

Footnotes

1. Bernstein, D and Lagakos, SW. Sample size and power determination for stratified clinical trials. Journal of Statistical Computation Simulation, 8:65–73, 1978. ↩

2. Nan: The original paper gives a FORTRAN routine. I found an R implementation. ↩