stand: Standardize covariate vector for balance constraint
In natstrat: Obtain Unweighted Natural Strata that Balance Many Covariates

This function is used by generate_constraints() to standardize covariate vectors to become balance constraints. The function divides the covariate values by the treated or average group standard deviation (across strata).

1	stand(z, x, denom_variance = "treated", treated = 1, autogen_missing = 50)

`z`	a factor with the `i`th entry equal to the treatment of unit `i`.
`x`	a covariate vector with `i`th entry equal to the covariate value of unit `i`. This should have the same order of units and length as `z`.
`denom_variance`	character stating what variance to use in the standardization: either the default "treated", meaning the standardization will use the treated variance (across all strata), where the treated group is declared in the `treated` argument, or "pooled", meaning the standardization will use the average of the variances of each treatment group.
`treated`	which treatment value should be considered the treated units. This must be one of the values of `z`.
`autogen_missing`	whether to automatically generate missingness constraint and how heavily to prioritize it. Should be a numeric or `NULL` value. `NULL` indicates that a constraint to balance the rate of missingness (denoted by `NA` in `x`) should not be automatically generated. Note that this is not recommended unless the user has already accounted for missing values. If not `NULL`, `autogen_missing` should be a numeric stating how heavily to prioritize generated missingness constraints over covariate constraint. The default is 50.

A list with two components:

covariate: a balance constraint for the standardized covariate values of all unites.
missingness: a corresponding balance constraint for the rate of missingness if autogen_missing not NULL, otherwise NULL.