Introduced error when calling even odd with just one factor. (Had to be an error rather than a warning because some later functions need the data to have length(factor) > 1)
Introduced warning when variables indicated by factor argument do not match the input data (x).
Use own warning for cases when NA arises because of 0 variance in even or odd vectors.
Fixes #22
I made some changes to evenodd:
length(factor) > 1
)