return residuals when running qmethod()

[maxheld83]

Maybe we want qmethod() to return the residuals by default?

psych::principal offers them.

Also seems like the centroid factor procedure in #229 returns this, so it might be nice to have it return it in all cases.

Might require validation vis-a-vis PQMethod (#210) before we do that, to keep qmethod() results pristine and validated 😄