Add attribute to drop levels in get method

[reinholdsson]

See the below example:

sum(a["SEKTORTILLHORIGH", 2010] == "Landsting" & rowSums(a["I38", c(2013, ._), na = 0]) > 1200)

sum(as.integer(a["SEKTORTILLHORIGH", 2010][[1]]) == 5 & rowSums(a["I38", c(2013, ._), na = 0]) > 1200)

The second query - where one use the ID instead of LEVEL - it's not that easy to use, an alternative could be:

sum(a["SEKTORTILLHORIGH", 2010, levels = F] == 5 & rowSums(a["I38", c(2013, ._), na = 0]) > 1200)

If this is to be implemented, we could add if (levels) ... in code. I guess header$type == "factor" variables shouldn't be converted from integer to factor as well.