Separate `confint` function

[smishr]

Right now #184 is work in progress to add confidence interval calculations.

@nadiaenh asked instead of multiple dispatch on summary statistics functions (mean, total, quantile etc), create new method for CI (say confint), and then dispatch it for different ::Function types.

Implementation wise, if the CI functionality is separately written to the basic summary stats function (ie mean(x,design) has different definition to mean(x,design, [CI kwargs ...])), then having a completely separate CI function may be logically simpler to work.

All changes to CI functionality will be in ci.jl, as opposed to spread over each of the summary stats functions.