ENH: basic handling of NaNs in resampling

Right now there is no clever imputation, just filling with zeros
ENH: NaN in resampling: extrapolation

extrapolation before resampling avoids completing with zeros, which has
side effects in the resampling.
ENH: warn properly on NaNs in resample
BUG: support old scikit-learn versions
DOC: better warning messages
BUG: fix old versions of scikit-learn
Style
Remove code duplication
WIP