Implement more accurate computation of very small LFSRs (and LFDRs) accurately

[pcarbo]

It could be useful in some cases to (accurately) compute very small LFSRs (e.g. < 1e-16) --- if you want to compare with very small p-values, for example. I'm not sure the existing implementation allows for that. (We would also need to come up with some good tests to evaluate these computations.)