In detrend.series.R:
- Warn if fit is not all positive (backup methods will be used)
- Use simpler but equivalent rules for checking validity of ModNegExp fit
- Small optimizations (rep.int; save and reuse length of input)


git-svn-id: svn://svn.r-forge.r-project.org/svnroot/dplr@724 edb9625f-4e0d-4859-8d74-9fd3b1da38cb