Add frequency weights to design.variables

[ayushpatnaikgit]

Since many functions require frequency weights, we can store it in design.variables rather than having each function derive it from probs.

Instead of probs, perhaps, we should call it probability_weights_reserved? Similarly, frequency_weights_reserved.

The reason we're only storing probability weights is that we didn't anticipate so many Julia functions to use frequency weights. I believe the survey package in R only store probability weights, and most R functions use probability weights.

Note, we want the information about weights to be in the same data frame and not make it a property of svydesign because of performance reasons. combine function in DataFrames works a lot faster.

[iuliadmtru]

I fixed this with PR #32, although the discussion should remain open regarding the names of the fields.

[ayushpatnaikgit]

I will discuss this with @chiraganand on phone.