Finite Population Corrections for `sem`

[ParadaCarleton]

When sampling from a finite population without replacement, the standard error is less than the current sem -- for instance, the standard error is 0 when you've sampled the whole population. It'd be nice to include some way to calculate the finite-population standard error.

[nalimilan]

Yes, we need a more general effort to support all kinds of survey statistics. This should probably go to a separate package, at least initially.