arrange asserts correct size. closes #155
Merge branch 'v0.2'
Fixes Date which can be both INTSXP and REALSXP.
Also includes lead and lag implementation, but not enabled yet because it does not handle all arguments yet.
hybrid impl of cumsum, plus generic impl of Mutater using CRTP. #133
hybrid impl of cumsum, plus generic impl of Mutater using CRTP. #133