Add analytical solutions with HT variance estimator

[smishr]

Right now mean total etc work with design::ReplicateDesign

We could add mean total etc with a Horwitz Thompson style estimator and variance using Hartley-Rao approximation.

We still dont know how double-inclusion probabilities are calculated from design information, so cant do Yates-Grundy or other more complex mathematical variance estimators.

Related to #85