select: on a categorical DataSeries should return a categorical DataSeries

``` Smalltalk
a := DataSeries withValues: #(1 2 2 3 5).
a makeCategorical.
a isCategorical. "true"

b := a select: [ :each | each = 2].
b isCategorical. "false"
```

When performing select: or reject: on a categorical DataSeries one should receive a categorical DataSeries (same with a numerical DataSeries)