Update dims functionality

- add dims argument to `cdb`, that set the default dimension value, e.g.

```
a <- cdb("db_path", dims = c(2012, 10))
```

- improve dims functionality in `[]` and `[]<-`, so that it only read variables that has the provided dimension, e.g.

```
# Write
a[ , "mtcars"] <- mtcars
a[ , "survey"] <- MASS::survey

# Read
a[ , "mtcars"]
a[ , "survey"]
```