[Rd] summary.lm fails for difftime objects

[Gabor Grothendieck]

lm works with difftime objects but then if you try to get the summary
it fails with an error:

  fit <- lm(as.difftime(Time, units = "mins") ~ demand, BOD)
  summary(fit)
  ## Error in Ops.difftime((f - mean(f)), 2) :
  ##  '^' not defined for "difftime" objects

A number of other lm methods also fail, e.g. plot(fit), but others
work, e.g. coef(fit), resid(fit)

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