WIP: Hartley Rao and other improvements

[smishr]

• Update docs with add Survey
• Add back Horvitz thompson total with Hartley Rao variance estimation
• Integrate HR with total with SurveyDesign
• Add some analytical formulae for total, deff and other srs formulae
• Add helper variance functions used in Taylor series linearisation
• Add tests

I couldnt get R to match Hartley Rao results like for like. R isnt using Hartley Rao algorithm everywhere, only for certain PPS cases. svyrecvar isnt the same as HR.

[ayushpatnaikgit]

We need to do something about tests. If we can't match answers with R, or any other language, it won't be good to merge this.

[codecov-commenter]

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[smishr]

We need to do something about tests. If we can't match answers with R, or any other language, it won't be good to merge this.

svyrecvar is a beast, but Im confident that a Julia native solution will be much faster, with less overheads.

Also Hartley Rao is good starting point. Better than returning a variance at all

[smishr]

@ayushpatnaikgit Should we

1. Integrate Hartley Rao variance with SurveyDesign. So that the four summary statistics return some variance (instead of nothing)
2. add tests here

Discussing and adding the Taylor series / recvar might take several months, as well as #277 and other big structural changes will take several months. Until then SurveyDesign will return HR by default? After that we can change default