makeWeightsDT: avoid column name conflicts

[WetRobot]

With bad luck, the names of e.g. values may coincide with the names of print variables. E.g. a Swede uses "obs" as a printing variable name (meaning "note"), and a value variable is named internally "obs".

> dt <- makeWeightsDT(ag2, print = quote(list(obs = sex, fot)), adjust = as, 
+                     values = list(quote(list(pyrs, obs = at.risk))), weights = ws,
+                     custom.levels = list(fot = fb))
Error in makeWeightsDT(ag2, print = quote(list(obs = sex, fot)), adjust = as,  : 
  Following column names duplicated (columns created by arguments print, adjust, etc.): 'obs'. 
If you see this, please ensure you are not passing e.g. the same column to both for adjusting 
and stratification (printing).

On the other hand it may be altogether bad practice to pass values with fixed names.