[Rd] coxph and drop1

[Terry Therneau]

A recent question in r-help made me realize that I should add a drop1 method 
for coxph and survreg.  The default does not handle strata() or cluster()
properly.  
  However, for coxph the right options for the "test" argument would be 
likelihood-ratio, score, and Wald; not chisq and F.  All of them reference
a chi-square distribution.  My thought is use these arguments, and add an
error message "read the help file for drop1.coxph" when the defaults appear.

  Any better suggestions?

Terry Therneau