design_sail: Sail design matrix
In sahirbhatnagar/sail: Sparse Additive Interaction Learning

Create design matrix used in sail function

1	design_sail(x, e, expand, group, basis, nvars, vnames, center.x, center.e)

`x`	input matrix of dimension `n x p`, where `n` is the number of subjects and p is number of X variables. Each row is an observation vector. Can be a high-dimensional (n < p) matrix. Can be a user defined design matrix of main effects only (without intercept) if `expand=FALSE`
`e`	exposure or environment vector. Must be a numeric vector. Factors must be converted to numeric.
`expand`	should `basis` be applied to every column of `x` (logical). Set to `FALSE` if you want a user defined main effects design matrix. If `FALSE` the `group` membership argument must also be supplied. Default: `TRUE`.
`group`	a vector of consecutive integers, starting from 1, describing the grouping of the coefficients. Only required when `expand=FALSE`.
`basis`	user defined basis expansion function. This function will be applied to every column in `x`. Specify `function(i) i` if no expansion is desired. Default: `function(i) splines::bs(i, df = 5)`.
`nvars`	number of variables
`vnames`	variable names
`center.x`	should the columns of `x` (after basis expansion) be centered (logical). Default: `TRUE`.
`center.e`	should exposure variable `e` be centered. Default: `TRUE`.