Proportion and count estimation for SimpleRandomSample and StratifiedSample

[smishr]

In R survey, population proportions and population total estimation is done by passing a 'factor' (CategoricalArray) variable to svymean svytotal (see pg 21 Lumley textbook).

Basic proportion and count estimation functionality was added to SimpleRandomSample and started for StratifiedSample, inside their respective svymean and svytotal functions to emulate functionality from R. This is currently done using an if statement

I am thinking that it would be neater and cleaner to have these features as multiple dispatch, instead of being caught by the if. Minor improvements in speed would also be there for very large datasets by using multiple dispatch.

[smishr]

#89 adds some of this functionality

[smishr]

#102 Adds some more features

[smishr]

Still need to add more testing in test/mean.jl and test/total.jl for CategoricalArray

[iuliadmtru]

Testing for CategoricalArray support is covered in issues #103 and #104.