Column descriptions interfere / get out-of-sync when adding/removing columns in matrixData

[jdrudolph]

Keeping track of column descriptions is not really possible at the moment. The following code will fail.

df <- matrixData(main=data.frame(a=1:3, b=6:8), description=c('a', 'b'))
main(df)['c'] <- 9:11

The only workaround currently is deleting all descriptions

description(df) <- character(0)

Maybe converting description to a list would make it easier to keep track of the column name -> description mapping. This is low priority for now. A test with warning was added in ae49bbe to keep track of this issue.