Remove previous lookup table on write

[reinholdsson]

When we overwrite an old variable, we need to make sure that there's no lookup table left over, for example when going from character to integer. We could remove the variable completely (clean), however, we need to figure out what to do when one only tries to replace one out of many dimensions (since the other dimensions still needs the lookup table). If we can't figure out any solution - we should probably not allow lookup tables for integer variables (which was implemented in #89).

library(Coldbir)
a_tmp <- tempfile()

N <- 1000
fun <- function(x) paste(sample(LETTERS, 10), collapse = "")
x <- sapply(1:N, fun)

a <- cdb(a_tmp)

# Write
a["x"] <- x

# Read
head(a["x"])
#1: UIQZDWPXVN
#2: NCDAOGRZVU
#3: FLJUSKEGQZ
#4: ICGHYQJPES
#5: BVAWNUEYOM
#6: OIYHZCBDUT

x <- sample(1:1000, N, replace = T)
# Write
a["x"] <- x

# Read
head(a["x"])
#1: CAMNKYOZWS
#2: ANZBRTDGSM
#3: QBKODRSJHG
#4: QERBLKIOTV
#5: HRFUQMIEWT
#6: NXATGZSIOU