Allow dataset subsetting with positional Arguments

[meggart]

Positional argument subsetting works for YAXArrays but does not for Datasets, this should be fixed. An MWE is:

using YAXArrays
import DimensionalData as DD
using Dates


tax = DD.Ti(Date(2001):Day(1):Date(2001,1,10))
ds = Dataset(gpp = YAXArray((tax,),rand(10)))
ds[DD.Ti(DD.Between(Date(2001,1,5),Date(2001,1,7)))] #error
ds.gpp[DD.Ti(DD.Between(Date(2001,1,5),Date(2001,1,7)))] #works