Internal helpers to project out one or more grouping factors from a
set of numeric columns before the main dcce() unit loop runs.
When a single factor is supplied the within transformation reduces to
a simple group-mean demeaning; for two or more factors the alternating
projections algorithm of Guimaraes & Portugal (2010) / Correia (2016)
is used.
The implementation is pure base R and requires no additional package dependencies.