Update data.table.R

 If .SDcols is a character but variables don't exist in the data.table, understand .SDcols  as wildcard.

```R
N=1e4; K=100
set.seed(1)
DT <- data.table(
  id = 1:N,
  v1 =  sample(5, N, TRUE),                          # int in range [1,5]
  v2 =  sample(1e6, N, TRUE),                        # int in range [1,1e6]
  v3 =  sample(round(runif(100,max=100),4), N, TRUE) # numeric e.g. 23.5749
)
DT[,colMeans(.SD),.SD="id v1"]
DT[,colMeans(.SD),.SD="id v*"]
DT[,colMeans(.SD),.SD=-"v*"]
DT[,colMeans(.SD),.SD=-"v?"]
````


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