Optimize everything for speed

[gasparl]

Although the POSSA internals are very fast and there seems to be little else to help speed, it would be great to somehow make the simulations faster.

For example, one way might be to have an option to provide total population once to the function (for each given sample), so that, instead of each time calling a sampling function, the population can be simply sampled in base R. Then again, the speed gain is probably not much with this, because the most time-consuming part is the statistical testing, which however seems inevitable.