expand `demean` to `degroup`

Some ideas for expanding the demean function into a more general degroup (or decenter, or ??) function (listed by ease of implementation as I perceive it):

1. Allow for group-centering around other functions. Popular choices I've seen: median(), min(), max(), also Mode() is popular for categorical predictors.

Mode <- function(x, multimodal = FALSE) {
  uniqv <- unique(x)
  tab <- tabulate(match(x, uniqv))
  if (multimodal) {
    idx <- which(tab==max(tab))
  } else {
    idx <- which.max(tab)
  }
  uniqv[idx]
}

2. Allow for more than 1 grouping var
   Order of operations would be: split y by G1, then split y_between by G2, etc... (Would need a better naming scheme?)

3. Center around an indexed value. For example, center y around y[time==0], or y[condition=="a"]. Can be mixed with (1): max(y[time==0]), etc.

Start by reading the existing demean function and its surrounding documentation and tests. Compare the proposed centering functions, multiple grouping variables, and indexed reference values, then clarify which scope and naming should be accepted. Done means one agreed API is implemented with documented behavior and coverage for its supported cases.